CBQE-819: rename purge/delete queue methods

Current purge queue method in rabbit_helper actually deletes the queue.
This change replaces it with the proper purging functionality and
makes a separate method for deletes.

Change-Id: I83cad7b47c12940bb5f2de72eaf5e1f8da355f9f
Reviewed-on: http://review.couchbase.org/24516
Tested-by: Tommie McAfee <tommie@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>