Add stream-level pushback into n23 design
In #4485 (moved) we identified what looks to be a flaw in the n23 design: since n23 only regulates how many cells can be in-flight on the circuit, it has no way to inform the other side that a given stream is blocked on writing and the other side should stop sending cells for that stream.
Options include something like the current stream-level sendmes (which would be a shame), or some sort of xoff/xon interface, or some other smart idea.
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