Remove old-style (direct) TCP replication support

[richvdh]

the old direct-to-master TCP replication protocol has been deprecated since Synapse 1.18.0 (nearly a year and a half, as of this writing).

We still have a bunch of code to support it (DirectTcpReplicationClientFactory etc), and it's still mentioned in various bits of documentation (in particular https://matrix-org.github.io/synapse/develop/tcp_replication.html needs an update). Having multiple ways to do it is confusing and hard to maintain, so we should remove it properly.

[richvdh]

( https://matrix-org.github.io/synapse/develop/tcp_replication.html claims "Currently all streams are written by a single process", which is no longer true. )

[reivilibre]

Notes from Patrick

1. You have to deal w/ removing things from the config. Are these now errors to config, or things to just ignore? (Note that our upgrade guide says "Hey just leave the old config settings they'll be ignored if you used Redis!")
2. There are worker docs to update.
3. It is slightly intertwined with Redis stuff / just touches a lot of stuff.

Sounds to me like step 1 might be altering documentation to forget about TCP replication. We can then remove the then-undocumented TCP replication 😇.

[clokep]

A big stopper to this is that many of the tests still assume TCP replication. I took a look at this and it isn't easy to untangle. 😢

There's a BaseStreamTestCase and a BaseMultiWorkerStreamTestCase, they have a lot of duplicate code between them but I think only the second one actually uses redis. I got a decent ways there by copying and pasting some code around, but still had a handful of failures that I would probably need @erikjohnston to help me figure out.

[clokep]

More concrete steps of what to do for this:

1. Update unit tests to not use the TCP replication classes (instead using the Redis ones). See BaseStreamTestCase. -- Reduce the number of tests using TCP replication #13543
2. Update sytest so that the Synapse runners use Redis instead of TCP replication. -- Always use Redis with Synapse workers. sytest#1282
3. Update Synapse CI to always use Redis for workers. -- Remove configuration options for direct TCP replication. #13647
4. Remove the ability to configure TCP replication listeners. -- Remove configuration options for direct TCP replication. #13647
5. Remove all the TCP replication protocol code.
6. Rename some things which refer to TCP replication but are also used by Redis. -- Rename get_tcp_replication. #12192
7. Update documentation. -- Remove configuration options for direct TCP replication. #13647

[clokep]

Update unit tests to not use the TCP replication classes (instead using the Redis ones). See BaseStreamTestCase.

@erikjohnston and I started some work on this in the clokep/test-redis branch. There are some inline notes of what needs to be done, but the gist is that reconnecting to Redis doesn't seem to be working properly and something is going wrong with our fake transports and such. 😢