clear up read-only cachedVBStates for unit tests
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 <333a3f71d3ad6e01fc953c4fa06bc1f3cd014910@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

hisundar/ep-engine,couchbase/ep-engine,couchbase/ep-engine,owendCB/ep-engine,membase/ep-engine,couchbase/ep-engine,jimwwalker/ep-engine,owendCB/ep-engine,daverigby/kv_engine,daverigby/ep-engine,membase/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,jimwwalker/ep-engine,hisundar/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,owendCB/ep-engine,teligent-ru/ep-engine,sriganes/ep-engine,sriganes/ep-engine,sriganes/ep-engine,daverigby/ep-engine,sriganes/ep-engine,jimwwalker/ep-engine,jimwwalker/ep-engine,daverigby/kv_engine,membase/ep-engine,daverigby/ep-engine,membase/ep-engine,hisundar/ep-engine,owendCB/ep-engine,daverigby/ep-engine,daverigby/kv_engine,hisundar/ep-engine,teligent-ru/ep-engine