MEDIUM: clock: don't compute before_poll when using monotonic clock

MEDIUM: clock: don't compute before_poll when using monotonic clock

There's no point keeping both clocks up to date; if the monotonic clock
is ticking, let's just refrain from updating the wall clock one before
polling since we won't use it. We still do it after polling however as
we need a wall clock time to communicate with outside.

This saves one gettimeofday() call per loop and two timeval comparisons.