This implements a more standard API for instantiating a homeserver and
moves some of the dependency injection into the test suite.

More concretely this stops using `setattr` on all `kwargs` passed to `HomeServer`.

This is so we can tell what is going on when things are taking a while to start up.

The main change here is to ensure that transactions that are created during startup get correctly logged like normal transactions.

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.

By persisting the user interactive authentication sessions to the database, this fixes
situations where a user hits different works throughout their auth session and also
allows sessions to persist through restarts of Synapse.

When running the UTs against a postgres deatbase, we need to set the collation
correctly.

* Pull Sentinel out of LoggingContext

... and drop a few unnecessary references to it

* Factor out LoggingContext.current_context

move `current_context` and `set_context` out to top-level functions.

Mostly this means that I can more easily trace what's actually referring to
LoggingContext, but I think it's generally neater.

* move copy-to-parent into `stop`

this really just makes `start` and `stop` more symetric. It also means that it
behaves correctly if you manually `set_log_context` rather than using the
context manager.

* Replace `LoggingContext.alive` with `finished`

Turn `alive` into `finished` and make it a bit better defined.

This is so that we don't have to rely on pulling it out from `current_state_events` table.

This encapsulates config for a given database and is the way to get new
connections.

The expected use case is to suppress MAU limiting on small instances

Python will return a tuple whether there are parentheses around the returned values or not.

I'm just sick of my editor complaining about this all over the place :)

The `expire_access_token` didn't do what it sounded like it should do. What it
actually did was make Synapse enforce the 'time' caveat on macaroons used as
access tokens, but since our access token macaroons never contained such a
caveat, it was always a no-op.

(The code to add 'time' caveats was removed back in v0.18.5, in #1656)

* Fix servlet metric names

Co-Authored-By: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Remove redundant check

* Cover all return paths

- Put the default window_size back to 1000ms (broken by #5181)
- Make the `rc_federation` config actually do something
- fix an off-by-one error in the 'concurrent' limit
- Avoid creating an unused `_PerHostRatelimiter` object for every single
  incoming request

* Pull config_dir_path and data_dir_path calculation out of read_config_files

* Pass config_dir_path and data_dir_path into read_config

Set default room version to v4.

Sends password reset emails from the homeserver instead of proxying to the identity server. This is now the default behaviour for security reasons. If you wish to continue proxying password reset requests to the identity server you must now enable the email.trust_identity_server_for_password_resets option.

This PR is a culmination of 3 smaller PRs which have each been separately reviewed:

* #5308
* #5345
* #5368

Migrate all tests to use the dict-based config format instead of hanging items off HomeserverConfig (#5171)

Collect all the things that make room-versions different to one another into
one place, so that it's easier to define new room versions.

I don't have a database with the same name as my user, so leaving the database
name unset fails.

While we're at it, clear out some unused stuff in the test setup.