[#3297] PerfMonMgr::processPktEventStack tests

[#3297] PerfMonMgr::processPktEventStack tests

src/hooks/dhcp/perfmon/monitored_duration_store.*
    Retooled to use composite key instead of DurationKey operators

src/hooks/dhcp/perfmon/perfmon_mgr.cc
    Altered some exception throws

src/hooks/dhcp/perfmon/tests/monitored_duration_store_unittests.cc
    TEST_F(MonitoredDurationStoreTest, adjacentEvent)
    TEST_F(MonitoredDurationStoreTest, adjacentEvent6) - new tests
    to check adjacent event ordering

src/hooks/dhcp/perfmon/tests/perfmon_mgr_unittests.cc
    TEST_F(PerfMonMgrTest4, invalidProcessPktEventStack)
    TEST_F(PerfMonMgrTest6, invalidProcessPktEventStack)
    TEST_F(PerfMonMgrTest4, processPktEventStack)
    TEST_F(PerfMonMgrTest6, processPktEventStack) - new tests