Add support for persisting event format versions

Add a 60s timeout to filtered room directory queries

MatrixFederationAgent: Look up the right SRV record

Rather than hardcoding a config which we always forget to update, generate it
from the default config.

This could cause exceptions if the id server returned 4xx responses.

Neilj/fix threepid auth check

Clarify docs for public_baseurl

This is leading to problems with people upgrading to clients that
support MSC1730 because people have this misconfigured, so try
to make the docs completely unambiguous.

The problem here is that we have cut-and-pasted an impl from Twisted, and then
failed to maintain it. It was fixed in Twisted in
https://github.com/twisted/twisted/pull/1047/files; let's do the same here.

Add metric for user dir current event stream position

Move SRV magic into an Agent-like thing

In the debian package, make the virtualenv symlink python to /usr/bin/python3.X
rather than /usr/bin/python3. Also make sure we depend on the right python3.x
package.

This might help a bit with subtle failures when people install a package from
the wrong distro (https://github.com/matrix-org/synapse/issues/4431).

Currently we only have the one event format version defined, but this
adds the necessary infrastructure to persist and fetch the format
versions alongside the events.

We specify the format version rather than the room version as:

1. We don't necessarily know the room version, existing events may be
   either v1 or v2.
2. We'd need to be careful to prevent/handle correctly if different
   events in the same room reported to be of different versions, which
   sounds annoying.

* Remove unnecessary ORDER BY clause

* Add logging

* Newsfile

this makes it easier to stub things out for tests.

it is only ever a bytes now, so let's enforce that.

this thing is now redundant.

... instead of the matrix_federation_endpoint

Pull the magic that is currently in matrix_federation_endpoint and friends into
an agent-like thing