State res v2 across large data sets can be very CPU intensive, and if
all the relevant events are in the cache the algorithm will run from
start to finish within a single reactor tick. This can result in
blocking the reactor tick for several seconds, which can have major
repercussions on other requests.

To fix this we simply add the occaisonal `sleep(0)` during iterations to
yield execution until the next reactor tick. The aim is to only do this
for large data sets so that we don't impact otherwise quick resolutions.=

It was originally implemented by pulling the full auth chain of all
state sets out of the database and doing set comparison. However, that
can take a lot work if the state and auth chains are large.

Instead, lets try and fetch the auth chains at the same time and
calculate the difference on the fly, allowing us to bail early if all
the auth chains converge. Assuming that the auth chains do converge more
often than not, this should improve performance. Hopefully.

The state res v2 algorithm only cares about the difference between auth
chains, so we can pass in the known common state to the `get_auth_chain`
storage function so that it can ignore those events.

... and use it in places where it's trivial to do so.

This will make it easier to pass room versions into the FrozenEvent
constructors.

When we perform state resolution, check that all of the events involved are in
the right room.

When we perform state resolution, check that all of the events involved are in
the right room.

Collect all the things that make room-versions different to one another into
one place, so that it's easier to define new room versions.

* Add helpers for getting prev and auth events

This is in preparation for allowing the event format to change between
room versions.