Use clock_gettime(2) on FreeBSD instead of gettimeofday(2).

Problem:
    gettimeofday(2) on FreeBSD synchronizes all the CPU clocks to ensure a
    monotonically increasing result. This causes pipeline flushes and delays
    on some CPUs which can be very damaging to performance on busy systems.

Analysis:
    Instead of gettimeofday(2), call clock_gettime(2) using the
    CLOCK_REALTIME_FAST option. This results in the same behavior as Linux's
    gettimeofday(2) and avoids the issues with clock synchronization.

Unit Tests: Pass.