Test and fix support for cancellation in Linearizer

A set of improvements to the Limiter

Linearizer was effectively a Limiter with max_count=1, so rather than
maintaining two sets of code, let's combine them.

* give them names, to improve logging
* use a deque rather than a list for efficiency

Fixes #3570

Switch changes to markdown & flip content on for miscs

(mostly just clarifications)

This reverts commit 21d3b879.

Changed http links to https

... we've fixed the things that caused the warnings, so we should reinstate the
warning.

Disable logcontext warning

Run things as background processes

Temporary workaround to #3518 while we release 0.33.0.

Add response code to response timer metrics

add config for pep8

Since, for better or worse, we seem to have configured isort to generate
89-character lines, pycharm is now complaining at me that our lines are too
long.

So, let's configure pep8 to behave consistently with flake8.

This is more involved than it might otherwise be, because the current
implementation just drops its logcontexts and runs everything in the sentinel
context.

It turns out that we aren't actually using a bunch of the functionality here
(notably suppress_failures and the fact that Distributor.fire returns a
deferred), so the easiest way to fix this is actually by simplifying a bunch of
code.

This fixes #3518, and ensures that we get useful logs and metrics for lots of
things that happen in the background.

(There are certainly more things that happen in the background; these are just
the common ones I've found running a single-process synapse locally).