MB-8992: Unlock a locked key when deleting with the proper cas

Change-Id: I057c290c40301fb4cd8667d7752abf2cfd778690
Reviewed-on: http://review.couchbase.org/28645
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>