MB-11576 Reserve cached Doc,DeleteCounts for safe read-only use

Dynamically allowing the cachedDocCount and cachedDeleteCount maps
to grow can be unsafe in the lock-less couch-kvstore implementation.
Pre-allocating their sizes to max number of vbucket ensures that we do
not cause a crash since the accesses should all be read-only.

Change-Id: I2ee48bfad7a8c5d4f14cb058a348b186b14cf156
Reviewed-on: http://review.couchbase.org/38894
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>