Use a monotonic clock on OS X

OS X doesn't have clock_gettime so we use something OS X-specific.
The clock_get_time is documented as monotonically increasing and the
clock_set_time function (which could screw that up) always returns
a KERN_FAILURE:

https://opensource.apple.com/source/xnu/xnu-2422.1.72/osfmk/kern/clock_oldops.c

Note the alternative AbsoluteTime API documented here

http://developer.apple.com/library/mac/qa/qa1398/

has been deprecated since 10.8.

Signed-off-by: David Scott <dave.scott@citrix.com>