`.user_id` is proxed to `.sender` in FrozenEvent, so this has no
functional change

This is so that everything is done in one place, making it easier to
change the event format based on room version

Split up event validation between event and builder

This bug was introduced in PR #4470, commit 678a92cb

* fix obvious problem :|

* changelog

Refactor event signing to work on dicts

two reasons for this. One, it saves a bunch of boilerplate. Two, it squashes
unicode to IDNA-in-a-`str` (even on python 3) in a way that it turns out we
rely on to give consistent behaviour between python 2 and 3.

Pass through room version to event auth

* load cert

* changelog

* fix

The validator was being run on the EventBuilder objects, and so the
validator only checked a subset of fields. With the upcoming
EventBuilder refactor even fewer fields will be there to validate.

To get around this we split the validation into those that can be run
against an EventBuilder and those run against a fully fledged event.

This is in preparation for making EventBuilder format agnostic, which
means event signing should be done against the event dict rather than
the EventBuilder object.

Migrate direct message and tag state on room upgrade

* Remove --process-dependency-links from UPGRADE.rst

Signed-off-by: Aaron Raimist <aaron@raim.ist>

* Add changelog

Signed-off-by: Aaron Raimist <aaron@raim.ist>

Refactoring in MatrixFederationAgent

We don't want to be doing .well-known lookups on these guys.

This is going to get too big and unmanageable.

Turns out that the library does a better job of parsing URIs than our
reinvented wheel. Who knew.

There are two things going on here. The first is that, unlike
parse_server_name, URI.fromBytes will strip off square brackets from IPv6
literals, which means that it is valid input to ClientTLSOptionsFactory and
HostnameEndpoint.

The second is that we stay in `bytes` throughout (except for the argument to
ClientTLSOptionsFactory), which avoids the weirdness of (sometimes) ending up
with idna-encoded values being held in `unicode` variables. TBH it probably
would have been ok but it made the tests fragile.

Require event format version to parse or create events

Disable native upsert on sqlite

If you use double-quotes here, you have to escape your backslashes. It's much
easier with single-quotes.

(Note that the existing double-backslashes are already interpreted by python's
""" parsing.)

Move the Host header logic down here so that (a) it is used if we reuse the
agent elsewhere, and (b) we can mess about with it with .well-known.