Add a mechanism to wake-up an external eventloop when a timer is inserted

[Matthias247]

The next_expiration API for the GenericTimerService doesn't work well in a multithreaded context. If an eventloop works like:

loop {
    let expiration = timer.next_expiration();
    let events = wait_for_events(expiration);
    handle_events(events);
}

it might miss the expiry of a timer if that one gets registered while the loop is already blocked in wait_for_events with a longer timeout.

Therefore the only reliable way to drive a timer in a multithreaded context is currently to poll the timer in regular intervals independent of the number of registered timers.

To fix this issue GenericTimerService could provide a hook that allows to wakeup the eventloop if a new timer gets registered whose expiry data is earlier than the previously registered timer.