Reduce impact of user callbacks to socket timer

The library should update socket timer after calling user functions to
exclude this time from timeout

Change-Id: Ie0297e7d7fd855c6dab84610a1f1fb7d5111ffb0
Reviewed-on: http://review.couchbase.org/23661
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>