Consistent naming: db.system to db.system.name, namespace constants, remove db from system-specific names

[lmolkova]

Fix #1581

Addresses DB-related portion of #608, replaces db-related things from #1711 and #1613.

1. Rename db.system to db.system.name. It gives us a good opportunity to clean up constants, batching multiple breaking changes together (see below)
2. Removes deprecated constants from new enum and updates existing constants to follow consistent naming
3. Removes db prefix from system-specific attributes following Add system-specific naming guidance #1708:
   • some systems don't fit into one domain (e.g. redis is a cache and database and also a messaging system)
   • not all operations that are done on the database are related to DB (control plane, auth, scaling, node communication)
   • we're quite inconsistent in naming
4. Adjusts _ -> . where it makes sense

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[trask]

I think I'm coming around to your proposal in #608 (comment)