An attempt at #401 - removing TX busywait

[Eugeny]

No description provided.

[EpicEric]

I don't see a way where the changes to the window size are communicated back to the watchers.

Otherwise this might work (I'm also not very familiar with low-level async), although something like Notify might be a better fit as it only lets in one task at a time, which is already the case given the Mutex for the window size.

[Eugeny]

Dang, I wasn't testing it correctly in the first place, thanks for the fix!
I've wrapped the window size updates in a fn so that the notifier fires after the update has already happened just in case there might be an await point in the future between the two.

I'm also wondering whether notify_all is needed instead of notify_one as a freshly awakened writer can be dropped before it is polled again.

[EpicEric]

Good point, that seems like the safer solution. Although that might cause unnecessary waking up of all writers.

I wonder if dropping a writer should invoke notify_one instead to avoid that (seemingly minor) issue.

[Eugeny]

@EpicEric is there anything you'd like to see added here?

[EpicEric]

This looks good to me.