MB-5844 disallow bucket names that start from

_replicator.couch. and _users.couch.

this resolves the situation when bucket directory name
conflicts with couchdb internal files _replicator.couch.1
and _users.couch.1 that happen to be located in the same
directory

Change-Id: I57318757e2d4b64d6da6e8a874e3d9ef5759d939
Reviewed-on: http://review.couchbase.org/27870
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>