Avoid N+1 circuit queries when fetching App Proxy worker information

The current Manager → App Proxy API includes the worker listing & detail queries that runs an N+1 worker-circuit queries, which may increase tail latency of the Manager and App Proxy APIs with a large number of circuits.

**We need to avoid this kind of queries and update Control Panel and Manager NOT TO RELY ON individual circuit information fetched like this.**

Instead, we have to:

- Offload or optimize the search for vacant circuits. (e.g., indexing based on Radix Tree?)
- Shrink the scope of circuit list queries to individual endpoints.
- Split worker information queries and circuit list queries. Use the latter sparingly as much as possible.

To keep App Proxy API compatibility, we could just let the `coordinator.models.worker.Worker.list_slots()` method to return an empty list. Afterwards, we MUST REDESIGN the App Proxy coordinator API.

JIRA Issue: BA-1985