CBQE-986: add persistence wait for pre-merge cases

Change-Id: Ic30c0e766a4d99fda7560bca66e141f165d9461a
Reviewed-on: http://review.couchbase.org/24202
Reviewed-by: Deepkaran Salooja <deepkaran.salooja@globallogic.com>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>