Fix hrtime on OSX

We had a chunk of dead code in there before. This commit removes the
dead code and replaces it with a more correct version.

Change-Id: I67a95b8bf9241250314504d690d8263ec1054cfa
Reviewed-on: http://review.couchbase.org/43538
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>