Since `RDY 1` was already set, once the next message comes in after we've `.success()` enough times to get out of a backoff state then a `RDY N` will be sent and we're back to normal.

Alternatively, I could keep an indicator of whether the current message FIN got us out of a backoff state and then return the RDY state to normal in this `_client_callback` function.  The main reason I didn't isn't because I wanted to keep all of the RDY related code in one place.