Consider a rate-limiting strategy: stress_small_msg is flaky

Consider:

- sending back a choke message if we're running out of send channel buffer on receiving messages, or
- expanding the permit system to include single block messages as well, or
- some other possibly just straight up sender-side rate-limit solution?

So that we can slow down / provide back-pressure to the sender and not have to close connections because we are receiving too fast.