Add *experimental* config option to support multiple databases

[erikjohnston]

Most of this is a refactor to remove the get_db_pool and get_db_conn functions from the homeserver god object.

Based on #6240 and #6245.

Commits independently reviewable.

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The new options replace database config, and look like:

databases:
    main_db:
        name: psycopg2
        args:
            database: pg1
    state_db:
        name: psycopg2
        args:
            database: pg2

data_stores:
    main: main_db
    state: state_b

I'm not super happy about this config format, hence why its undocumented and experimental, but its useful to allow us to test. Once I've played around with it a bit more, possibly adding support having multiple state data stores, then it should be easy to change or stabilise.

[erikjohnston]

I need to figure out what to do about mypy, as I don't think its feasible to fix all the errors there in one go (especially since it looks like it would also requiring documenting some of our deps)

I've disabled the follow import stuff for now. We weren't using it before and it seems a bit overly aggressive.

[richvdh]

I want to take this out of the queue until some of the groundwork is in place (and the conflicts are fixed...)