[#3297] Verify enable-monitoring and cleanup

[#3297] Verify enable-monitoring and cleanup

src/hooks/dhcp/perfmon/perfmon_callouts.cc
    pkt4_send(CalloutHandle& handle)
    pkt6_send(CalloutHandle& handle) - move query check, log
     and enable check into processPktEventStack()

src/hooks/dhcp/perfmon/perfmon_mgr.cc
    PerfMonMgr::processPktEventStack() - do log, query check,
    and enable check

src/hooks/dhcp/perfmon/tests/perfmon_mgr_unittests.cc
    testProcessPktEventStack() - updated to verify
    disabling monitoring inhibits duration data storage