So this basically means that replay throttling relies on the recipient being well-behaved in not acknowledging delivery twice; can there be a race between ACK and NACK for the same message?

My intuition is that this might want to be changed to actually track which messages are outstanding instead of using a counter—unless I am missing something wrt. how to make that counter be reliable in all cases (which does mean that `math.max()` must be unnecessary, for example).