Fixes #10325

We might have events in the staging area if the service was restarted while there were unhandled events in the staging area.

Fixes #10295

Fix link in modules doc to point at instructions on registering a callback instead of ones on registering a web resource.

Instead of mixing them with user authentication methods.

this was a typo introduced in #10282. We don't want to end up doing the
`replace_stream_ordering_column` update after anything that comes up in
migration 60/03.

Now that we've fixed it.

Related to #10251

The presence router docs include some sample homeserver config. At some point we changed the name of the [config option](https://github.com/matrix-org/synapse/blob/859dc05b3692a3672c1a0db8deaaa9274b6aa6f5/docs/sample_config.yaml#L104-L113), but forgot to update the docs.

I've also added `presence.enabled: true` to the example, as that's the new way to enable presence (the `presence_enabled` option has been deprecated).

We need to rebuild *all* of the indexes that use the current `stream_ordering`
column.

This broke in #10272

Fixes #9490

This will break a couple of SyTest that are expecting failures to be added to the response of a federation /send, which obviously doesn't happen now that things are asynchronous.

Two drawbacks:

    Currently there is no logic to handle any events left in the staging area after restart, and so they'll only be handled on the next incoming event in that room. That can be fixed separately.
    We now only process one event per room at a time. This can be fixed up further down the line.

This adds a simple best effort locking mechanism that works cross workers.

Previously m.child.room events in non-space rooms would be
treated as part of the room graph, but this is no longer
supported.

It was possible for us not to find any rows in a batch, and hence conclude that
we had finished. Let's not do that.

* Move background update names out to a separate class

`EventsBackgroundUpdatesStore` gets inherited and we don't really want to
further pollute the namespace.

* Migrate stream_ordering to a bigint

* changelog