MB-12137: Don't update the current snapshot during persistence

If we update the current snapshot only when we persist items to
disk then we may miss out on updates to the current snapshot
when there are vbucket state changes.

Change-Id: Iea3139afb669bdf32e6b4d98e8b3515cafe578dc
Reviewed-on: http://review.couchbase.org/41289
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>