Partial rewrite to try to address shortcomings.

This version:

* Has a few more features to do with room management, based on the other
  matrix-nio backend
* Removes the 'users are rooms' hack in favour of message overriding
* Adds support for management rooms, giving bots the ability to message
  first/send 'direct' messages
* Fixes a deadlock caused by mixing async and non-async code (bad idea)
* Adds some more debug logging to help track down crashes in the async
  parts of the code