MB-11731: Vbucket-level locking as opposed to shard-level

For flusher tasks:
- flushVBucket
- compactVBucket
- snapshot task
- vbDelete

Change-Id: I0f17ed7dacdc23556f110e271048836ed6701685
Reviewed-on: http://review.couchbase.org/34501
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>