A few different implementations of computing the current time were
spread out through the code base, most of them using gettimeofday().
This centralizes the logic in CUtil::GetTime() for easier maintenance,
and also allows all call sites to get the benefit of the clock_gettime()
code path on systems that support it.
The gettimeofday function returns 0 for success, not for failure. As a
result of the inverted logic we were losing millisecond precision when
parsing incoming messages on non-HAVE_CLOCK_GETTIME systems (macOS).
