[#3257] Suppress NCRs when reusing leases

[#3257] Suppress NCRs when reusing leases

/src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::createNameChangeRequests() - modified to check for
    reused lease

/src/bin/dhcp4/tests/dora_unittest.cc
    DORATest::leaseCaching() - modified to verify suppression of NCRs

/src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::createNameChangeRequests() - modified to iteralte over
    IA contexts rather than IA options in response and to check for
    reused leases

/src/bin/dhcp6/tests/fqdn_unittest.cc
    Updated tests to populate IAContexts

/src/bin/dhcp6/tests/sarr_unittest.cc
    SARRTest::leaseCaching()- modified to verify suppression of NCRs

/src/lib/dhcpsrv/alloc_engine.*
    AllocEngine::ClientContext6::IAContext - added reused_leases_ container
    AllocEngine::ClientContext6::getIAContexts() - new function