[#3297] Initial Packet processing implemented

[#3297] Initial Packet processing implemented

PerfMonMgr can now process packet event stacks,
poplating duration store and driving alarm checking
Not yet tied to pkt_send handlers

src/hooks/dhcp/perfmon/monitored_duration.*
    DurationKey::getStatName()
                 setSubnetId() - new functions

src/hooks/dhcp/perfmon/perfmon_messages.mes
    PERFMON_ALARM_CLEARED
    PERFMON_ALARM_TRIGGERED - new messages

src/hooks/dhcp/perfmon/perfmon_mgr.*
    PerfMonMgr::init() - new function

    PerfMonMgr::processPktEventStack()
    PerfMonMgr::reportToStatsMgr()
    PerfMonMgr::reportAlarm() - implemented

src/hooks/dhcp/perfmon/tests/monitored_duration_unittests.cc
    Updated to test getStatName()

src/hooks/dhcp/perfmon/tests/perfmon_mgr_unittests.cc
    class TestablePerfMonMgr : public PerfMonMgr - new
    Expanded PerfMonMgrTest with new tests functions

    TEST_F(PerfMonMgrTest4, testReportToStatsMgr)
    TEST_F(PerfMonMgrTest6, testReportToStatsMgr)
    TEST_F(PerfMonMgrTest4, testAddDurationSample)
    TEST_F(PerfMonMgrTest6, testAddDurationSample)
    - new tests