This is the first tranche of support for room versioning. It includes:
 * setting the default room version in the config file
 * new room_version param on the createRoom API
 * storing the version of newly-created rooms in the m.room.create event
 * fishing the version of existing rooms out of the m.room.create event

This reverts commit ec716a35.

The field is never read from, and all the opportunities given to populate it are not utilized. It should be very safe to remove this.

Because it was complicated and annoyed me. I suspect this will be more
efficient too.

on_notifier_poke no longer runs synchonously, so we have to do a different hack
to make sure that the replication data has been sent. Let's actually listen for
its arrival.