MB-11731 Add the daemon vbucket state snapshot task.

This new daemon task will be scheduled periodically and persist
any vbucket state changes with a lower priority.

Change-Id: I2aa7dc5dda0153aac74314e239a85ad3f444c4d8
Reviewed-on: http://review.couchbase.org/39907
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>