Ensure good comprehension hygiene using flake8-comprehensions.

* Raise an exception if there are pending background updates

So we return with a non-0 code

* Changelog

* Port synapse_port_db to async/await

* Port update_database to async/await

* Add version string to mocked homeservers

* Remove unused imports

* Convert overseen bits to async/await

* Fixup logging contexts

* Fix imports

* Add a way to print an error without raising an exception

* Incorporate review

This encapsulates config for a given database and is the way to get new
connections.

This adds:

* a test sqlite database
* a configuration file for the sqlite database
* a configuration file for a postgresql database (using the credentials in `.buildkite/docker-compose.pyXX.pgXX.yaml`)

as well as a new script named `.buildkite/scripts/test_synapse_port_db.sh` that:

1. installs Synapse
2. updates the test sqlite database to the latest schema and runs background updates on it
3. creates an empty postgresql database
4. run the `synapse_port_db` script to migrate the test sqlite database to the empty postgresql database (with coverage)

Step `2` is done via a new script located at `scripts-dev/update_database`.

The test sqlite database is extracted from a SyTest run, so that it can be considered as an actual homeserver's database with actual data in it.

Add a linting script that enforces all boolean values in the default config be lowercase.

This has annoyed me for a while so I decided to fix it.

This doesn't work, and afaict hasn't been used since 2015.

Add a dev script to cover all the different linting steps.

* py3 fixes for federation_client
* .well-known support for federation_client

* Fix check-newsfragment script

I previously broke this so that it always succeeded...

* more fixes

* fix newsfiles

* You need an entry in the debian changelog (and not a regular newsfragment)
  for debian packaging changes.
* Regular newsfragments must end in full stops.

Rewrite this in python so that it can be run in parallel.

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.

Signed-off-by: Schnuffle <schnuffle@github.com>

* changelog

* reduce circleci config

* plus a handy script

* fix regex

The problem with this script is that it is largely untested, entirely
unmaintained, and running it is likely to make your synapse blow up in
exciting ways.

For example, it leaves a bunch of tables with dead values in it, like
event_to_state_groups.

Having it here sends a message that it is a supported part of
synapse, which is absolutely not the case.

This appears to have stopped working since matrix.org moved to cloudflare. The
Host header should match the name of the server, not whatever is in the SRV
record.