Remove explicit key parameter on Timer

[Max-Meldrum]

Currently, it is possible to give the key to the timer. This should be set by the runtime.

One timer per (key, timestamp) data point.

pub fn schedule_at<I: Into<u64>>(
        &mut self,
        key: I,
        time: u64,
        entry: TimerState,
) -> TimerResult<TimerState> {
   self.timer.schedule_at(key.into(), time, entry)
}