Don't poll in `as_generated`.

[RedRoserade]

Description

I noticed that as_generated did polling via asyncio.sleep. I realized that this can be improved by instead making use of one single helper queue that keeps track of what the tailers produce. So, this changeset makes use of a single queue with value wrappers to signal to the queue consumer what to do. It also prevents polling, which could make this more efficient.

Fixes: Nothing