MB-11731 Replace the shard-level vbstate snapshot with individual tasks.

As we replace a shard-level lock with a vbucket-level lock to synchronize
the flusher, vbucket deletion, vbucket compaction, vbucket state
persistence tasks, this changes also replaces the shard-level vbstate
snapshot task with individual vbstate persistence tasks.

Change-Id: Ief7138a4b63d388cd786b966f68fcd8ab6ce2722
Reviewed-on: http://review.couchbase.org/39906
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>