Improve performance of getting typing updates for replication

Build and push docker image to hub automatically

Clarifications in FederationHandler

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

Latest is horrible and makes debugging what has happened anywhere a
nightmare. We push a latest because of demand for it, but we'll also
push a SHA1 commit id so those wanting to know what they're running
(and be able to roll back if required) can use those instead.

Note that latest here is defined as "most recent master commit" not
"most recent released version", as the actual semantics of making latest
correct while still being able to build bugfixed releases of previous
versions is just ARGH. So we define it as "master" not "latest release".

* add some comments on things that look a bit bogus
* rename this `state` variable to avoid confusion with the `state` used
  elsewhere in this function. (There was no actual conflict, but it was
  a confusing bit of spaghetti.)

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)