[#3253] Added in-place duration update

[#3253] Added in-place duration update

src/hooks/dhcp/perfmon/monitored_duration.*
    MonitoredDuration::MonitoredDuration() - new copy ctor
    DurationDataInterval::operator==() - added for UTs

src/hooks/dhcp/perfmon/monitored_duration_store.*
    MonitoredDurationStore::addDurationSample()
    - new function for adding sample to duration in-place
    MonitoredDurationStore::addDuration()
    - removed optional sample parameter

src/hooks/dhcp/perfmon/tests/monitored_duration_store_unittests.cc
    addDurationSampleTest(uint16_t family)  - new test func and tests

src/hooks/dhcp/perfmon/tests/monitored_duration_unittests.cc
    TEST(MonitoredDuration, copyConstructors) - new test