mcreq: move mcreq_sched_* functions to mcreq.h

Implement the lcb_* APIs in instance.c

Change-Id: I0484f3266acc9dba21d389cbec00d75ef9e19119
Reviewed-on: http://review.couchbase.org/37570
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>