MB-11863 DaemonVBSnapshot task must complete before shutdown

Change-Id: Iacb7852525a5055bac16a61d02e4ed65daf17319
Reviewed-on: http://review.couchbase.org/40155
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>