util.smqueue: Refactor checkpoint logic and add tests The previous checkpoint logic used recursion, and the code was more complex than needed due to maintenance of the self._next_checkpoint field. This switches to a loop, which should be a bit more efficient, partly because there are fewer calls (although it was a tail call) and because table lookups can now be cached between each execution. _call_checkpoints() is now called unconditionally for every ack, which is simpler though adds a function call, it should be more than balanced out by the other changes.

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