OrderedOutputBuffer could handle backpressure to avoid overflowing memory

[Yomguithereal]

This would of course potentially slow everything down and could lead to nasty deadlocks if not implemented properly.

[Yomguithereal]

I think it does not slow down because of backpressure but ramp up memory usage. This is because task_done is called before the item is actually yielded downstream (which makes sense else it would incur some lock until the sink has finished processing an item. But we could give this power to the user to ensure atomicity wrt sink)