The broadcast method is unsafe

[andersea]

The way broadcasting to taps and extensions works currently is not safe. It should support backpressure, but currently messages just pile up in a big queue, leading to possible memory leaks and other unpleasant issues. This needs a thorough rethink. There is a lot of discussion available on the issue of broadcasting in the trio community as well.

See for instance: https://github.com/python-trio/trio/issues/987