mcreq: Free queue array on cleanup

Plugs a memory leak when the pipelines array was not freed

Change-Id: Ia0a0dc347ede504eabe8c3079d30e5824b2e04c2
Reviewed-on: http://review.couchbase.org/35870
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>