operations: add mcreq_sched_enter() usage

Even though this function is currently a noop, it's still good to have

Change-Id: Ib141b8c4573e8b8c6ca31696fc64d627ee4ad7a5
Reviewed-on: http://review.couchbase.org/37786
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>