MB-9974: Refactor couch_upr:process_item/3

Two places, same code which is soon needed more often, i.e. refactor.

Change-Id: Ia7fe84c63bcd45cbf24f1fec5628aa8b54d52391
Reviewed-on: http://review.couchbase.org/33747
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>