Stabilize the threads API (MSC3856) by supporting (only) the v1
path for the endpoint.
This also marks the API as safe for workers since it is a read-only
API.
Implement the /threads endpoint from MSC3856.
This is currently unstable and behind an experimental configuration
flag.
It includes a background update to backfill data, results from
the /threads endpoint will be partial until that finishes.