Co-Authored-By: Erik Johnston <erik@matrix.org>

This reverts commit 11c23af4.

So we can debug issues like #6683 more easily

Fix #6727
Related #6655

Co-authored-by: Erik Johnston <erikj@jki.re>

Fix instantiation of message retention purge jobs

Log saml assertions rather than the whole response

Turns out that figuring out a remote user id for the SAML user isn't quite as obvious as it seems. Factor it out to the SamlMappingProvider so that it's easy to control.

This will be used to retry outbound transactions to a remote server if
we think it might have come back up.

Generally try to make this more comprehensible, and make it match the
conventions.

I've removed the documentation for all the settings which allow you to change
the names of the template files, because I can't really see why they are
useful.

... since the whole response is huge.

We even need to break up the assertions, since kibana otherwise truncates them.

* Port synapse.replication.tcp to async/await

* Newsfile

* Correctly document type of on_<FOO> functions as async

* Don't be overenthusiastic with the asyncing....

When figuring out which topological token to start a purge job at, we
need to do the following:

1. Figure out a timestamp before which events will be purged
2. Select the first stream ordering after that timestamp
3. Select info about the first event after that stream ordering
4. Build a topological token from that info

In some situations (e.g. quiet rooms with a short max_lifetime), there
might not be an event after the stream ordering at step 3, therefore we
abort the purge with the error `No event found`. To mitigate that, this
patch fetches the first event _before_ the stream ordering, instead of
after.

Cleanups and additions to the module API

Allow REST endpoint implemnentations to raise a RedirectException, which will
redirect the user's browser to a given location.

Currently we rely on `current_state_events` to figure out what rooms a
user was in and their last membership event in there. However, if the
server leaves the room then the table may be cleaned up and that
information is lost. So lets add a table that separately holds that
information.

This means that things like to device messages don't get blocked behind
processing PDUs, which can potentially take *ages*.