dnsdist: Make the backend queryLoad and dropRate values atomic

dnsdist: Make the backend queryLoad and dropRate values atomic

So that there is no race when these vaues are read by the SNMP or
web threads and updated by the health check thread at the same time.
Reported by Thread Sanitizer:

WARNING: ThreadSanitizer: data race (pid=11167)
  Write of size 8 at 0x7b7400002558 by thread T18:
    #0 healthChecksThread() /opt/project/pdns/dnsdistdist/dnsdist.cc:1712:22 (dnsdist+0xf2a4a2)
    #1 void std::__invoke_impl<void, void (*)()>(std::__invoke_other, void (*&&)()) /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/invoke.h:60:14 (dnsdist+0xf40ea2)
    #2 std::__invoke_result<void (*)()>::type std::__invoke<void (*)()>(void (*&&)()) /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/invoke.h:95:14 (dnsdist+0xf40e0d)
    #3 decltype(std::__invoke(_S_declval<0ul>())) std::thread::_Invoker<std::tuple<void (*)()> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/thread:244:13 (dnsdist+0xf40dd5)
    #4 std::thread::_Invoker<std::tuple<void (*)()> >::operator()() /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/thread:253:11 (dnsdist+0xf40d95)
    #5 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (*)()> > >::_M_run() /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/thread:196:13 (dnsdist+0xf40c29)
    #6 <null> <null> (libstdc++.so.6+0xbbb2e)

  Previous read of size 8 at 0x7b7400002558 by thread T2:
    #0 backendStatTable_handler(netsnmp_mib_handler_s*, netsnmp_handler_registration_s*, netsnmp_agent_request_info_s*, netsnmp_request_info_s*) /opt/project/pdns/dnsdistdist/dnsdist-snmp.cc:356:62 (dnsdist+0xeccf98)
    #1 netsnmp_call_next_handler <null> (libnetsnmpagent.so.30+0x2a0cc)
    #2 SNMPAgent::handleSNMPQueryCB(int, boost::any&) /opt/project/pdns/dnsdistdist/snmp-agent.cc:96:13 (dnsdist+0xfb0847)
    #3 boost::detail::function::void_function_invoker2<void (*)(int, boost::any&), void, int, boost::any&>::invoke(boost::detail::function::function_buffer&, int, boost::any&) /usr/include/boost/function/function_template.hpp:118:11 (dnsdist+0x8937d8)
    #4 boost::function2<void, int, boost::any&>::operator()(int, boost::any&) const /usr/include/boost/function/function_template.hpp:768:14 (dnsdist+0xf9d56c)
    #5 EpollFDMultiplexer::run(timeval*, int) /opt/project/pdns/dnsdistdist/epollmplexer.cc:176:7 (dnsdist+0xfd041b)
    #6 SNMPAgent::worker() /opt/project/pdns/dnsdistdist/snmp-agent.cc:141:24 (dnsdist+0xfb0b6f)
    #7 void std::__invoke_impl<void, void (SNMPAgent::*)(), SNMPAgent*>(std::__invoke_memfun_deref, void (SNMPAgent::*&&)(), SNMPAgent*&&) /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/invoke.h:73:14 (dnsdist+0xf344c9)
    #8 std::__invoke_result<void (SNMPAgent::*)(), SNMPAgent*>::type std::__invoke<void (SNMPAgent::*)(), SNMPAgent*>(void (SNMPAgent::*&&)(), SNMPAgent*&&) /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/invoke.h:95:14 (dnsdist+0xf343c1)
    #9 decltype(std::__invoke(_S_declval<0ul>(), _S_declval<1ul>())) std::thread::_Invoker<std::tuple<void (SNMPAgent::*)(), SNMPAgent*> >::_M_invoke<0ul, 1ul>(std::_Index_tuple<0ul, 1ul>) /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/thread:244:13 (dnsdist+0xf3436e)
    #10 std::thread::_Invoker<std::tuple<void (SNMPAgent::*)(), SNMPAgent*> >::operator()() /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/thread:253:11 (dnsdist+0xf34315)
    #11 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (SNMPAgent::*)(), SNMPAgent*> > >::_M_run() /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/thread:196:13 (dnsdist+0xf34079)
    #12 <null> <null> (libstdc++.so.6+0xbbb2e)