there's really no point spinning up two separate jobs for these.

I don't think we ever use this, and it slows things down. If we want to use it,
we should just do so on a couple of builds rather than all of them.

(We really don't need to kick off 10 builds for the branch as well as 10 for
the PR)

Allow backslashes in event field filters

Fix a number of flake8 errors

Add v2 state resolution algorithm

Fixes a bug introduced in https://github.com/matrix-org/synapse/pull/1783 which
meant that single backslashes were not allowed in event field filters.

The intention here is to allow single-backslashes, but disallow
double-backslashes.

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.

Clean up the way logcontexts and threads work in the pushers

Clean up room alias creation

... and rename it, for even more sanity

Handle HttpResponseException more safely for federated groups

Wrap calls to deferToThread() in a thing which uses a child logcontext to
attribute CPU usage to the right request.

While we're in the area, remove the logcontext_tracer stuff, which is never
used, and afaik doesn't work.

Fixes #4064

Add information about the matrix-docker-ansible-deploy playbook

We don't do anything with the result, so this is needed to give this code a
logcontext.

This brings it into line with on_new_notifications and on_new_receipts. It
requires a little bit of hoop-jumping in EmailPusher to load the throttle
params before the first loop.

`on_new_notifications` and `on_new_receipts` in `HttpPusher` and `EmailPusher`
now always return synchronously, so we can remove the `defer.gatherResults` on
their results, and the `run_as_background_process` wrappers can be removed too
because the PusherPool methods will now complete quickly enough.

Each pusher has its own loop which runs for as long as it has work to do. This
should run in its own background thread with its own logcontext, as other
similar loops elsewhere in the system do - which means that CPU usage is
consistently attributed to that loop, rather than to whatever request happened
to start the loop.

simplifies the interface to _start_pushers

... and use it from start_pusher_by_id. This mostly simplifies
start_pusher_by_id.

This is public (or at least, called from outside the class), so ought to have a
better name.

Run MAU queries as background processes

Make psutil an explicit dependency

Fixes #3820

As of #4027, we require psutil to be installed, so it should be in our
dependency list. We can also remove some of the conditional import code
introduced by #992.

Fixes #4062.