disable direct access to _replicator database

Change-Id: Idda001840564fe184614aa085f564e8cf3682543
Reviewed-on: http://review.couchbase.org/33149
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>