MB-11794 ensure cachedVBStates is read-only safe

Preallocate cachedVBStates to ensure that there are no runtime
modifications which may cause any concurrent read-only operations to crash
Change-Id: I4b3df2834226919864cf69ae40557815b890f211
Reviewed-on: http://review.couchbase.org/39834
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>