Add mc_REQDATAPROCS for extended routines needed for 'complex' commands

This will help future extensibility and uniformity when custom handling
is required.

Change-Id: Idc391287373d8998300cc72c1e4da7d1d08ad7f2
Reviewed-on: http://review.couchbase.org/41610
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>