The idea is to engineer it so that the vast majority of the rooms can
stay in the cache, so we can just ignore them.

The decorator assumed we were always wrapping function methods

That file was getting long.

The changes are non functional, and simply split things up into:
- the main class
- the connection store
- the extensions
- the types

Let's only pull out the events we care about. Note that the index isn't
necessary here, as postgres is happy to scan the set of rooms for the
events.

This should let us see how quickly we actually process things in
practice.

This supersedes #17503, given the per-connection state is being heavily
rewritten it felt easier to recreate the PR on top of that work.

This correctly handles the case of timeline limits going up and down.

This does not handle changes in `required_state`, but that can be done
as a separate PR.

Based on #17575.

---------

Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>

This is to make initial sliding sync a bit faster

Add connection tracking to the receipts extension.

Based on #17574

---------

Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>

This is some prep work ahead of correctly tracking receipts, where we
will also want to track the room status in terms of last receipt we had
sent down.

Essentially, we add two classes `PerConnectionState` and a mutable
version, and then operate on those.

---------

Co-authored-by: Eric Eastwood <eric.eastwood@beta.gouv.fr>

Results in:

```
AssertionError: null
  File "synapse/http/server.py", line 332, in _async_render_wrapper
    callback_return = await self._async_render(request)
  File "synapse/http/server.py", line 544, in _async_render
    callback_return = await raw_callback_return
  File "synapse/federation/transport/server/_base.py", line 369, in new_func
    response = await func(
  File "synapse/federation/transport/server/federation.py", line 826, in on_GET
    await self.media_repo.get_local_media(
  File "synapse/media/media_repository.py", line 473, in get_local_media
    await respond_with_multipart_responder(
  File "synapse/media/_base.py", line 353, in respond_with_multipart_responder
    assert content_length is not None
```

This stops people from getting half way through a step and it failing
due to the github token having expired (this happens to me every damn
time).

Follow on from #17537.

This is just adding a batched lookup function (you might want to hide
whitespace in the diff).

So that clients can check for support. Note that if the feature is only
enabled for some users, the `/versions` request must be authenticated to
pick up that SSS is enabled for the user

`old_verify_keys` isn't marked as required in
https://spec.matrix.org/v1.11/server-server-api/#get_matrixkeyv2server
and there's no functional difference between an empty object and
omitting the object, so I don't think there's any reason synapse should
explode when the field is omitted.

Follows on from #17567

Follow on from #17558

Basically, we want to reduce the number of threads we want to use at a
time, i.e. reduce the number of threads that are paused/blocked. We do
this by returning from the thread when the consumer pauses the producer,
rather than pausing in the thread.

---------

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

Previously, we just had very basic partial room exclusion based on
whether we were lazy-loading room members. Now with this PR, we added
`must_await_full_state(...)` with rules to check if we have a we're only
requesting `required_state` which is completely satisfied even with
partial state.

Partially-stated rooms should have all state events except for remote
membership events so if we require a remote membership event anywhere,
then we need to return `True`.

This is really spurious and causes a lot of spam. I don't think there is
a use for it even at DEBUG level.

Regression from #17558

We do this by reading from a threadpool, rather than blocking the main
thread.

This is broadly what we do in the [S3 storage
provider](https://github.com/matrix-org/synapse-s3-storage-provider/blob/main/s3_storage_provider.py#L234)