Improve TimerManager API to allow removing specific task

The TimerManager::remove() method removes all timers with the specified callback, and does so by traversing the list of timers.

This should be improved by returning a "handle" in `TimerManager::add`, and supporting efficiently removing a single timer from its handle:

class TimerManager {
     Timer add(shared_ptr<Runnable> task, const struct timeval& value);
     void remove(Timer t);
}