Rename channel's `async close()` methods to `aclose()`

[llucax]

What's needed?

In Python there are already a few async close methods, like for async generators. It also includes a aclosing context manager to automatically close these objects.

Proposed solution

See sub-issues.

[shsms]

But we have the same issue again. If we aclose, it ideally shouldn't return until the channel's buffer is drained. But that's not how it is implemented now. Changing the existing method to a sync close makes more sense.

[llucax]

Yeah, that could be an alternative if it makes sense, but the at least the broadcast channel needs to acquire a condition variable to notify all receivers, so it needs to be async.