More responsive canceling of reports

When you run `python3 -m crystalfontz.dbus.client listen` and try to `^C` it, there's a long delay, and you often have to send the signal twice.

I believe this is because of the use of async iterators, which don't yield out of the event loop when there are no signals. I suspect that with some *really* clevering refactoring, this could be improved.