Detect conflicting timer rescheduling

Detect conflicting timer rescheduling

Repeating a timer can be done in two ways:

 * Returning true from the handler
 * Calling start() again in the handler

The latter is useful if you want to change the timer interval.

If both are used, then it becomes ambiguous when the timer should fire
again.

Detect this case and warn about it. Current implementation will respect
the new interval given to start(), rather than the interval set before
running the handler.