Changing the output timestamp of a timer does not clear the previously set timer

While looking into an unrelated bug with GroupIntoBatches, I noticed that it seems like changing the output timestamp of a timer does not clear the existing timer, and instead creates a new one.  

This kind of makes sense looking at the implementation of timers in Dataflow, the output timestamp is encoded into the timer ID, but this is not reflected in the timerStillPresent map in WindmillTimerInternals.  It seems like it should be, and the previous timer should be deleted.