Use C++11 to implement gethrtime

Change-Id: I1a11ca271ed4b290e34190e3a1c35be48dec80f4
Reviewed-on: http://review.couchbase.org/31771
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>