TimerService wrapper:
* Can schedule many tasks at once.
* When scheduled for 1000ms, may run in 1000ms, 10001ms, etc.
* Cancel returns true => future task guaranteed cancelled;
        returns false => already cancelled || already run.
* Scheduling a new task is O(log(N)) where N is # active tasks.
* Cancelling a task is O(N).
* Timer-service thread will kill itself automatically when no task is
  scheduled for 4 minutes, and will restart automatically when a new task
  is added.