mcreq: add_pipelines should copy pipeline list

This makes it simpler to add extra data at the end of the pipeline list,
if necessary

Change-Id: I5e4a6be67f58d340cd0d99850fef7e5ae2f3d32e
Reviewed-on: http://review.couchbase.org/39877
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>