Fetch events in a separate transaction.

This has a couple of benefits:

- It reduces the time of transactions, allowing other database requests
  to run.
- Fetching events is given a dedicated database thread, and so can't
  starve other database requests.

Move /sync state calculations from rest to handler

Pass make_guest when we autogen a user ID

Pull guest access token out of the auth session params

Switch over /events to use per room caches

Pull guest access token out of the auth session params, otherwise it will break if you open the email on a different device.

Cache get_room_changes_for_user

Only use room_ids if in get_room_events_stream if is_guest

Cache fixes

If stream pos is greater then earliest known key and entity hasn't changed, then entity hasn't changed

Prefill stream change caches

Make /events always return a newer token, if one exists

Fix up the /account/3pid API

Add config option for setting the trusted id servers, disabling checking the ID server in integration tests