mcreq: add mcreq_first_packet()

Change-Id: I64639033ef6d6c62c7c9b34b9ed81a525b9dbbd4
Reviewed-on: http://review.couchbase.org/36078
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>