MB-8413 Release KVShard instances upon a bucket deletion.

Change-Id: I91db6eb20c30a37df98b23b7ac8799d0cbff7e64
Reviewed-on: http://review.couchbase.org/26846
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>