Consider using other mach_*_time() functions for monotonic time on OSX

As of OSX 10.9, there is a mach_approximate_time() which looks at first glance like it might be suitable for use as our monotime_coarse.

As of OSX 10.12, there are mach_continuous_absolute_time() and mach_continuous_approximate_time(), which are supposed to keep counting while the system is sleeping.