Fix the regression from the change for MB-11943

The change for MB-11943 caused the sporadic failure
in "vbucket deletion doesn't affect new data" unit test case.

This change makes sure that we have the same logic in maintaining
cachedVBState instance for a deleted vbucket.

Change-Id: I7fda66a258c46cd04820001e945e0679eed0469b
Reviewed-on: http://review.couchbase.org/40645
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>