Password-protected keyspaces use RBAC not SASL

SASL is a thing of the past, RBAC is our current security mechanism.

SASL-protected buckets have been turned silently into RBAC buckets – what happens is that there is one RBAC user created with the same name of the bucket.

The creds parameters are still honoured by cbq and cbq-engine, so in general terms it is possible to send a query with multiple credentials, one for each keyspace.

The keyspace could be a collection or bucket, it doesn't matter.

Change the term "SASL-protected bucket" (or similar) to "password-protected keyspace".