[#3328] Output usecs; add permon-get-all-durations

[#3328] Output usecs; add permon-get-all-durations

Statistics and raw data return durations in microseconds instead
of milliseconds.  Add initial support for perfmon-get-all-durations.

/doc/sphinx/arm/hooks-perfmon.rst
    Updated to microseconds
    Updated perfmon-control command

/src/hooks/dhcp/perfmon/monitored_duration.*
    DurationKey::toElement()
    MonitoredDuration::toElement() - new functions

/src/hooks/dhcp/perfmon/perfmon_callouts.cc
    int perfmon_control()
    int perfmon_get_all_durations() - new functions

    int load() - register commands

/src/hooks/dhcp/perfmon/perfmon_config.cc
    PerfMonConfig::parse() - replace use of copy ctor

    PerfMonConfig::enable_monitoring_
    PerfMonConfig::stats_mgr_reporting_ - made std::atomic

/src/hooks/dhcp/perfmon/perfmon_messages.mes
    PERFMON_CMDS_CONTROL_ERROR
    PERFMON_CMDS_CONTROL_OK
    PERFMON_CMDS_GET_ALL_DURATIONS_ERROR
    PERFMON_CMDS_GET_ALL_DURATIONS_OK - new messages

/src/hooks/dhcp/perfmon/perfmon_mgr.cc
    PerfMonMgr::perfmonControlHandler()
    PerfMonMgr::perfmonGetAllDurationsHandler()
    PerfMonMgr::formatDurationDataAsElements()
    PerfMonMgr::formatDurationDataAsResultSet() - new functions

/src/hooks/dhcp/perfmon/tests/Makefile.am
    Added perfmon_cmds_unittests.cc

/src/hooks/dhcp/perfmon/tests/perfmon_config_unittests.cc
    Replaced use of copy ctor

/src/hooks/dhcp/perfmon/tests/perfmon_mgr_unittests.cc
    Updated tests for microseconds