The descs.Collection should mediate and maintain in-memory changes not just to descriptors, but also to anything joining on descriptors:

• namespace entries,
• zone configs,
• comments,
• is there anything else? idk.

This implies also caching in the stored layer, plumbing the reads of these new system tables into catkv.catalogQuerier, adding and adopting more accessor methods on the Collection, etc.

We can then get rid of the MetadataUpdater, CommentCache etc. interfaces and implementations.

At that point it should also be trivial to extend nstree.Catalog to include zone configs and comments.

Jira issue: CRDB-19972

Epic CRDB-19158