improve open system keyspaces throughput by caching positive privilege checks

Open system keyspaces (eg system:keyspaces) check privileges on every keyspace they scan.
The issue is that most likely, the user scanning the keyspaces would have been granted query_select[bucket:*:*], so checking for individual collections would be pointless once we have checked the bucket, and may result in extra cbauth calls.
Cache bucket and scope privilege checks.