This requires a new config option to specify which media repo should be
responsible for running background jobs to e.g. clear out expired URL
preview caches.

Implementation of https://github.com/matrix-org/matrix-doc/pull/2625

The aim here is to make it easier to reason about when streams are limited and when they're not, by moving the logic into the database functions themselves. This should mean we can kill of `db_query_to_update_function` function.

This ended up being a bit more invasive than I'd hoped for (not helped by
generic_worker duplicating some of the code from homeserver), but hopefully
it's an improvement.

The idea is that, rather than storing unstructured `dict`s in the config for
the listener configurations, we instead parse it into a structured
`ListenerConfig` object.

Fixes https://github.com/matrix-org/synapse/issues/7683

Broke in: #7649

We had a `yield` acting on a coroutine. To be fair this one is a bit difficult to notice as there's a function in the middle that just passes the coroutine along.

The spec [states](https://matrix.org/docs/spec/client_server/r0.6.1#phone-number) that `m.id.phone` requires the field `country` and `phone`.

In Synapse, we've been enforcing `country` and `number`.

I am not currently sure whether this affects any client implementations.

This issue was introduced in #1994.

Enable 3PID add/bind/unbind endpoints on r0 routes