time: use mach_absolute_time() for monotonic time

time: use mach_absolute_time() for monotonic time

* use mach_absolute_time() for monotonic time
  mach_absolute_time() is faster and monotonic, see
  https://developer.apple.com/library/content/qa/qa1398/_index.html

  for its implementation, see
  https://opensource.apple.com/source/xnu/xnu-3248.60.10/libsyscall/wrappers/mach_absolute_time.s

  it's using rdtsc.
* and remove unnecessary headers from ceph_time.h

Signed-off-by: Kefu Chai <kchai@redhat.com>