CBQE-0: use wait_for_persistence instead of waiting many stats

Change-Id: I67bd6541e8389186a2f49359a64d1aaa0631d3f5
Reviewed-on: http://review.couchbase.org/35067
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>