In almost every case, this won't matter. But the assumption is that it's
an Identifier subclass and there might be plugins that are using that
fact...

Invoke the bot callback in an async wrapper.

See merge request !1

Hopefully, should stop plugins blocking the async thread.

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