prod: explicitly log every state change

I worked out the formal state machines of the relevant goroutines and stuck a
log message in at every state change. This should help with debugging #179.

The logs should still only trigger on broker rebalance or super-heavy traffic,
so normal operation should be quiet.

@wvanbergen 
this was misleading because if the message had already been retried we would just flush it out the errors channel instead of requeueing it