Require event format version to parse or create events

Add room_version param to get_pdu

Store rejected remote invite events as outliers

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.

Merge branch 'erikj/fixup_rejecting_invites' of github.com:matrix-org/synapse into erikj/require_format_version

Currently they're stored as non-outliers even though the server isn't in
the room, which can be problematic in places where the code assumes it
has the state for all non outlier events.

In particular, there is an edge case where persisting the leave event
triggers a state resolution, which requires looking up the room version
from state. Since the server doesn't have the state, this causes an
exception to be thrown.