[time] Introduce zx_clock_get_monotonic

Rather than introduce a failure path into zx_clock_get simply to
report an invalid clock id, we will introduce distinct functions per
clock, for those clocks that cannot fail.

Test: wrote a test that ensures the monotonic clock is positive and
does not go backwards.

ZX-2187

Change-Id: I509710b163a5bbf4b436ad06507e6a16afef4f2b