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).

* Remove unnecessary ORDER BY clause

* Add logging

* Newsfile