Rewrite the dockerfile as a multistage build: this means we can get rid of a whole load of cruft which we don't need.

remove spurious federation checks on localhost

Remove redundant call to start_get_pdu_cache

I think this got forgotten in #3932. We were getting away with it because it
was the last call in this function.

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)

There's really no point in checking for destinations called "localhost" because
there is nothing stopping people creating other DNS entries which point to
127.0.0.1. The right fix for this is
https://github.com/matrix-org/synapse/issues/3953.

Blocking localhost, on the other hand, means that you get a surprise when
trying to connect a test server on localhost to an existing server (with a
'normal' server_name).

trivial fixes for docstring

* changelog

* reduce circleci config

* plus a handy script

* fix regex