Fixes https://github.com/matrix-org/synapse/issues/6583

This mostly applise to `*stateGroupCache*` and co.

Broke in #6391.

This is already correctly done when we instansiate the cache, but wasn't
when it got reloaded (which always happens at least once on startup).

This is a cherry-pick of 1a1da60a (#7470)
to the release-v1.13.0 branch.

Attempts to clarify the sample config for databases, and add some stuff about
tcp keepalives to `postgres.md`.

Extends #5794 etc to the SimpleHttpClient so that it also applies to non-federation requests.

Fixes #7092.

Ensure good comprehension hygiene using flake8-comprehensions.

Template config files

* Imagine a system composed entirely of x, y, z etc and the basic operations..

Wait George, why XOR? Why not just neq?

George: Eh, I didn't think of that..

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

- 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

test threepid checking

This allows specifying who and what is allowed to be published onto the
public room list

If TLS is disabled, it should not be an error if no cert is given.

Fixes #4554.

* remove dh_params and set better cipher string

Broadly three things here:

* disable W504 which seems a bit whacko
* remove a bunch of `as e` expressions from exception handlers that don't use
  them
* use `r""` for strings which include backslashes

Also, we don't use pep8 any more, so we can get rid of the duplicate config
there.

This is a mixed commit that fixes various small issues

 * print parentheses
 * 01 is invalid syntax (it was octal in py2)
 * [x for i in 1, 2] is invalid syntax
 * six moves

Signed-off-by: Adrian Tschira <nota@notafile.com>

https://github.com/matrix-org/synapse/pull/2755 broke log-config generation,
which in turn broke the unit tests.

Renames ``load_config`` to ``load_or_generate_config``
Adds a method called ``load_config`` that just loads the
config.

The main synapse.app.homeserver will continue to use
``load_or_generate_config`` to retain backwards compat.
However new worker processes can use ``load_config`` to
load the config avoiding some of the cruft needed to generate
the config.

As the new ``load_config`` method is expected to be used by new
configs it removes support for the legacy commandline overrides
that ``load_or_generate_config`` supports

Currently we store all access tokens in the DB, and fall back to that
check if we can't validate the macaroon, so our fallback works here, but
for guests, their macaroons don't get persisted, so we don't get to
find them in the database. Each restart, we generate a new ephemeral
key, so guests lose access after each server restart.

I tried to fix up the config stuff to be less insane, but gave up, so
instead I bolt on yet another piece of custom one-off insanity.

Also, add some basic tests for config generation and loading.