Better and more accurate timeout handling.

This patch attempts to provide more accuracy for timeouts, so that
commands are expired when their lifetime has reached; no sooner and no
later within a resolution of microseconds

Change-Id: Ib4fd47e65f9ed7614689fd3c5c86096b754c437a
Reviewed-on: http://review.couchbase.org/31755
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>