auth: Prevent a race during the processing of SVC auto-hints

auth: Prevent a race during the processing of SVC auto-hints

When `svc-autohints` is enabled, the content of SVCB and HTTPS records
is modified in `PacketHandler::doAdditionalProcessing()` to expand
the IPv4 and IPv6 with their actual values.
This causes an issue because the content of these records might be
shared between threads, via the record cache, and one thread could
be trying to read from the internal `std::set` while a second thread
is altering it, leading to a data race and possibly to memory corruption
and a crash.
This is correctly detected by TSAN:
```
WARNING: ThreadSanitizer: data race (pid=102795)
  Write of size 8 at 0x7b3400010350 by thread T33:
    #0 operator delete(void*) <null> (pdns_server+0x211b7c) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #1 std::__new_allocator<std::_Rb_tree_node<SvcParam>>::deallocate(std::_Rb_tree_node<SvcParam>*, unsigned long) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/new_allocator.h:158:2 (pdns_server+0x33fc78) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #2 std::allocator_traits<std::allocator<std::_Rb_tree_node<SvcParam>>>::deallocate(std::allocator<std::_Rb_tree_node<SvcParam>>&, std::_Rb_tree_node<SvcParam>*, unsigned long) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/alloc_traits.h:496:13 (pdns_server+0x33fc78)
    #3 std::_Rb_tree<SvcParam, SvcParam, std::_Identity<SvcParam>, std::less<SvcParam>, std::allocator<SvcParam>>::_M_put_node(std::_Rb_tree_node<SvcParam>*) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/stl_tree.h:565:9 (pdns_server+0x33fc78)
    #4 std::_Rb_tree<SvcParam, SvcParam, std::_Identity<SvcParam>, std::less<SvcParam>, std::allocator<SvcParam>>::_M_drop_node(std::_Rb_tree_node<SvcParam>*) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/stl_tree.h:632:2 (pdns_server+0x33fc78)
    #5 std::_Rb_tree<SvcParam, SvcParam, std::_Identity<SvcParam>, std::less<SvcParam>, std::allocator<SvcParam>>::_M_erase_aux(std::_Rb_tree_const_iterator<SvcParam>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/stl_tree.h:2495:7 (pdns_server+0x33fc78)
    #6 std::_Rb_tree<SvcParam, SvcParam, std::_Identity<SvcParam>, std::less<SvcParam>, std::allocator<SvcParam>>::erase[abi:cxx11](std::_Rb_tree_const_iterator<SvcParam>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/stl_tree.h:1197:2 (pdns_server+0x33fc78)
    #7 std::set<SvcParam, std::less<SvcParam>, std::allocator<SvcParam>>::erase[abi:cxx11](std::_Rb_tree_const_iterator<SvcParam>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/stl_set.h:655:21 (pdns_server+0x33fc78)
    #8 SVCBBaseRecordContent::setHints(SvcParam::SvcParamKey const&, std::vector<ComboAddress, std::allocator<ComboAddress>> const&) /work/pdns/pdns/dnsrecords.cc:768:14 (pdns_server+0x33fc78)
    #9 PacketHandler::doAdditionalProcessing(DNSPacket&, std::unique_ptr<DNSPacket, std::default_delete<DNSPacket>>&) /work/pdns/pdns/packethandler.cc:565:16 (pdns_server+0x4ed330) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #10 PacketHandler::doQuestion(DNSPacket&) /work/pdns/pdns/packethandler.cc:1794:5 (pdns_server+0x4f79b4) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #11 PacketHandler::question(DNSPacket&) /work/pdns/pdns/packethandler.cc:1175:10 (pdns_server+0x4f649a) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #12 MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::distribute(int) /work/pdns/pdns/./distributor.hh:220:14 (pdns_server+0x260f70) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #13 MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()::operator()() const /work/pdns/pdns/./distributor.hh:179:25 (pdns_server+0x260b31) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #14 void std::__invoke_impl<void, MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>(std::__invoke_other, MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:61:14 (pdns_server+0x260b31)
    #15 std::__invoke_result<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>::type std::__invoke<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>(MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:96:14 (pdns_server+0x260b31)
    #16 void std::thread::_Invoker<std::tuple<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>>::_M_invoke<0ul>(std::_Index_tuple<0ul>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:258:13 (pdns_server+0x260b31)
    #17 std::thread::_Invoker<std::tuple<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>>::operator()() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:265:11 (pdns_server+0x260b31)
    #18 std::thread::_State_impl<std::thread::_Invoker<std::tuple<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>>>::_M_run() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:210:13 (pdns_server+0x260b31)
    #19 execute_native_thread_routine /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:82:18 (libstdc++.so.6+0xd72c2) (BuildId: 6fe66a2d539a78c993bd2d377e00fad389220963)

  Previous read of size 2 at 0x7b3400010350 by thread T39:
    #0 SvcParam::getKey() const /work/pdns/pdns/./svc-records.hh:80:12 (pdns_server+0x3721f3) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #1 GenericDNSPacketWriter<std::vector<unsigned char, std::allocator<unsigned char>>>::xfrSvcParamKeyVals(std::set<SvcParam, std::less<SvcParam>, std::allocator<SvcParam>> const&) /work/pdns/pdns/dnswriter.cc:404:23 (pdns_server+0x3721f3)
    #2 void HTTPSRecordContent::xfrPacket<GenericDNSPacketWriter<std::vector<unsigned char, std::allocator<unsigned char>>>>(GenericDNSPacketWriter<std::vector<unsigned char, std::allocator<unsigned char>>>&, bool) /work/pdns/pdns/dnsrecords.cc:348:1 (pdns_server+0x3349bd) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #3 HTTPSRecordContent::toPacket(GenericDNSPacketWriter<std::vector<unsigned char, std::allocator<unsigned char>>>&) /work/pdns/pdns/dnsrecords.cc:348:1 (pdns_server+0x3349bd)
    #4 DNSRecordContent::serialize[abi:cxx11](DNSName const&, bool, bool) /work/pdns/pdns/./dnsparser.hh:215:11 (pdns_server+0x311140) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #5 DNSPacket::addRecord(DNSZoneRecord&&) /work/pdns/pdns/dnspacket.cc:177:68 (pdns_server+0x2fa894) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #6 PacketHandler::doAdditionalProcessing(DNSPacket&, std::unique_ptr<DNSPacket, std::default_delete<DNSPacket>>&) /work/pdns/pdns/packethandler.cc:542:8 (pdns_server+0x4eccf2) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #7 PacketHandler::doQuestion(DNSPacket&) /work/pdns/pdns/packethandler.cc:1794:5 (pdns_server+0x4f79b4) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #8 PacketHandler::question(DNSPacket&) /work/pdns/pdns/packethandler.cc:1175:10 (pdns_server+0x4f649a) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #9 MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::distribute(int) /work/pdns/pdns/./distributor.hh:220:14 (pdns_server+0x260f70) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #10 MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()::operator()() const /work/pdns/pdns/./distributor.hh:179:25 (pdns_server+0x260b31) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #11 void std::__invoke_impl<void, MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>(std::__invoke_other, MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:61:14 (pdns_server+0x260b31)
    #12 std::__invoke_result<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>::type std::__invoke<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>(MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:96:14 (pdns_server+0x260b31)
    #13 void std::thread::_Invoker<std::tuple<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>>::_M_invoke<0ul>(std::_Index_tuple<0ul>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:258:13 (pdns_server+0x260b31)
    #14 std::thread::_Invoker<std::tuple<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>>::operator()() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:265:11 (pdns_server+0x260b31)
    #15 std::thread::_State_impl<std::thread::_Invoker<std::tuple<MultiThreadDistributor<DNSPacket, DNSPacket, PacketHandler>::MultiThreadDistributor(int)::'lambda'()>>>::_M_run() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:210:13 (pdns_server+0x260b31)
    #16 execute_native_thread_routine /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:82:18 (libstdc++.so.6+0xd72c2) (BuildId: 6fe66a2d539a78c993bd2d377e00fad389220963)

  Thread T33 'pdns/distributo' (tid=102833, running) created by thread T17 at:
    #0 pthread_create <null> (pdns_server+0x1904e6) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #1 __gthread_create /usr/src/debug/gcc/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/gthr-default.h:663:35 (libstdc++.so.6+0xd73a9) (BuildId: 6fe66a2d539a78c993bd2d377e00fad389220963)
    #2 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State>>, void (*)()) /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:147:37 (libstdc++.so.6+0xd73a9)
    #3 Distributor<DNSPacket, DNSPacket, PacketHandler>::Create(int) /work/pdns/pdns/./distributor.hh:134:18 (pdns_server+0x256d23) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #4 qthread(unsigned int) /work/pdns/pdns/auth-main.cc:536:25 (pdns_server+0x256d23)
    #5 void std::__invoke_impl<void, void (*)(unsigned int), unsigned int>(std::__invoke_other, void (*&&)(unsigned int), unsigned int&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:61:14 (pdns_server+0x2635f0) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #6 std::__invoke_result<void (*)(unsigned int), unsigned int>::type std::__invoke<void (*)(unsigned int), unsigned int>(void (*&&)(unsigned int), unsigned int&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:96:14 (pdns_server+0x2635f0)
    #7 void std::thread::_Invoker<std::tuple<void (*)(unsigned int), unsigned int>>::_M_invoke<0ul, 1ul>(std::_Index_tuple<0ul, 1ul>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:258:13 (pdns_server+0x2635f0)
    #8 std::thread::_Invoker<std::tuple<void (*)(unsigned int), unsigned int>>::operator()() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:265:11 (pdns_server+0x2635f0)
    #9 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (*)(unsigned int), unsigned int>>>::_M_run() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:210:13 (pdns_server+0x2635f0)
    #10 execute_native_thread_routine /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:82:18 (libstdc++.so.6+0xd72c2) (BuildId: 6fe66a2d539a78c993bd2d377e00fad389220963)

  Thread T39 'pdns/distributo' (tid=102837, running) created by thread T19 at:
    #0 pthread_create <null> (pdns_server+0x1904e6) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #1 __gthread_create /usr/src/debug/gcc/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/gthr-default.h:663:35 (libstdc++.so.6+0xd73a9) (BuildId: 6fe66a2d539a78c993bd2d377e00fad389220963)
    #2 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State>>, void (*)()) /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:147:37 (libstdc++.so.6+0xd73a9)
    #3 Distributor<DNSPacket, DNSPacket, PacketHandler>::Create(int) /work/pdns/pdns/./distributor.hh:134:18 (pdns_server+0x256d23) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #4 qthread(unsigned int) /work/pdns/pdns/auth-main.cc:536:25 (pdns_server+0x256d23)
    #5 void std::__invoke_impl<void, void (*)(unsigned int), unsigned int>(std::__invoke_other, void (*&&)(unsigned int), unsigned int&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:61:14 (pdns_server+0x2635f0) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593)
    #6 std::__invoke_result<void (*)(unsigned int), unsigned int>::type std::__invoke<void (*)(unsigned int), unsigned int>(void (*&&)(unsigned int), unsigned int&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:96:14 (pdns_server+0x2635f0)
    #7 void std::thread::_Invoker<std::tuple<void (*)(unsigned int), unsigned int>>::_M_invoke<0ul, 1ul>(std::_Index_tuple<0ul, 1ul>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:258:13 (pdns_server+0x2635f0)
    #8 std::thread::_Invoker<std::tuple<void (*)(unsigned int), unsigned int>>::operator()() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:265:11 (pdns_server+0x2635f0)
    #9 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (*)(unsigned int), unsigned int>>>::_M_run() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:210:13 (pdns_server+0x2635f0)
    #10 execute_native_thread_routine /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:82:18 (libstdc++.so.6+0xd72c2) (BuildId: 6fe66a2d539a78c993bd2d377e00fad389220963)

SUMMARY: ThreadSanitizer: data race (/work/pdns-rgacogne/pdns/pdns_server+0x211b7c) (BuildId: 384adc19a67695435bd5e89d0a77f562561f4593) in operator delete(void*)
```

To prevent this issue, this commit wraps the internal `std::set` in a
mutex. In theory this will cause a performance impact, but in practice
I did not find it to be noticeable.
If we ever do, a different solution would be to duplicate the content
of the SVCB/HTTPS records before modifying them.