select: Fix timer handling

We're actually passed the relative usecs, so we need to ensure that
we mark the absolute begin time, and compare that to the post loop
timer. Previously we were comparing the relative timestamp.

Change-Id: I735f74d7f16773526338b345b02d694586c2a129
Reviewed-on: http://review.couchbase.org/29212
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>