clear up read-only cachedVBStates for unit tests

cachedVBStates need to be cleared up for the read-only instances
otherwise data from the previous unit test interferes with the current run

This will still not affect the read-write path where we protect
by pre-allocating the cachedVBStates

Change-Id: I5d628544ab528b334372217625e40f7c6fa0a443
Reviewed-on: http://review.couchbase.org/39855
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>