MillisTimer - handle begin when active

Two issues:

if you called begin or begingRepeating while still in waiting queue, it would create circular state (code did not set state to waiting)

If you called begin on one in active list, it would first remove it from list, but did not update the timings for items left as the timing all rely on items before it

teensy3/EventResponder.cpp

 	bool irq = disableTimerInterrupt();
 	_next = listWaiting;
 	listWaiting = this; // TODO: use STREX to avoid interrupt disable
+	_state = TimerWaiting;
 	enableTimerInterrupt(irq);
 }
 
 	if (s == TimerActive) {
 		if (_next) {
 			_next->_prev = _prev;
+			_next->_ms += _ms;   // add in the rest of our timing to next entry...
 		}
 		if (_prev) {
 			_prev->_next = _next;