The bane of async generators

[goodboy]

As per #56, discussion on gitter, and discussion in python-trio/trio#638 async generators aren't task safe and cause all sorts of subtle problems if used without some kind of higher level wrapping to conduct teardown, that is an explicit aclose() call. This issue is mostly to keep track of the long running discussion and eventually integrate whatever consensus solution is chosen.

For now I've implemented a custom ReceiveChannel as a stopgap but even it isn't sufficient to address premature stream break-ing/teardown currently. I would like to experiment with using an contextlib.AsyncExitStack to ensure, at least at the actor nursery level, aclose() is always called on all streams.

The effort on this general front includes quite a few issues worth watching:

• @njsmith's pep for deterministic iterator cleanup is still open, but is still without much traction
• the strategic discussion in Implement a strategy for handling (async) generator cleanup python-trio/trio#265
• some prior discussion in trio's 638