Run notify_app_services as a bg process

Replaced all occurences of e.message with str(e)

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

fix #3854 MAU transaction errors

Logging improvements

when processing incoming transactions, it can be hard to see what's going on,
because we process a bunch of stuff in parallel, and because we may end up
recursively working our way through a chain of three or four events.

This commit creates a way to use logcontexts to add the relevant event ids to
the log lines.

I spent ages trying to figure out how I was going mad...

fix docstring for FederationClient.get_state_for_room

Some logging tweaks to help with debugging incoming federation transactions

This ensures that its resource usage metrics get recorded somewhere rather than
getting lost.

(It also fixes an error when called from a nested logging context which
completes before the bg process)

trivial fixes for docstring

* changelog

* reduce circleci config

* plus a handy script

* fix regex

Fix ExpiringCache.__len__ to be accurate

Move synctl into top dir to avoid a symlink

Comments and interface cleanup for on_receive_pdu

It used to try and produce an estimate, which was sometimes negative.
This caused metrics to be sad, so lets always just calculate it from
scratch.

(This appears to have been a longstanding bug, but one which has been made more
of a problem by #3932 and #3933).

(This was originally done by Erik as part of #3933. I'm cherry-picking it
because really it's a fix in its own right)

make python 3 work in the docker container

Fix some instances of ExpiringCache not expiring cache items

Synapse doesn’t allow for media resources to be played directly from
Chrome. It is a problem for users on other networks (e.g. IRC)
communicating with Matrix users through a gateway. The gateway sends
them the raw URL for the resource when a Matrix user uploads a video
and the video cannot be played directly in Chrome using that URL.

Chrome argues it is not authorized to play the video because of the
Content Security Policy. Chrome checks for the "media-src" policy which
is missing, and defauts to the "default-src" policy which is "none".

As Synapse already sends "object-src: 'self'" I thought it wouldn’t be
a problem to add "media-src: 'self'" to the CSP to fix this problem.

Fix spurious exceptions when client closes conncetion

symlinks apparently break setuptools on python3 and alpine
(https://bugs.python.org/issue31940), so let's stop using a symlink and just
use the file directly.

We require attrs 16.0.0

Ref: https://github.com/matrix-org/synapse/issues/3945

Given we have disabled lazy loading for incr syncs in #3840, we can make self-LL more efficient by only doing it on initial sync.  Also adds a bounds check for if/when we change our mind, so that we don't try to include LL members on sync responses with no timeline.