MB-11943: replace map with vector for cachedVBStates

std::map can incur thread-unsafe rb balance operations which
may invalidate concurrent iterators causing asserts or crashes
Pre-allocated vectors are thread-safe as long as the size is not
altered
Change-Id: I777b0cc7d9c421cc11425db75506fdf90044a93c
Reviewed-on: http://review.couchbase.org/40600
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>