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).

Resource tracking for background processes

Remove event re-signing hacks

Comment dummy TURN parameters in default config

This introduces a mechanism for tracking resource usage by background
processes, along with an example of how it will be used.

This will help address #3518, but more importantly will give us better insights
into things which are happening but not being shown up by the request metrics.

We *could* do this with Measure blocks, but:
 - I think having them pulled out as a completely separate metric class will
   make it easier to distinguish top-level processes from those which are
   nested.

 - I want to be able to report on in-flight background processes, and I don't
   think we want to do this for *all* Measure blocks.

Fix visibility of events from erased users over federation