Request for Actors to be notified when the runtime is stopped

[lovettchris]

ICoyoteRuntime.Stop() does not tell the actors about this event, it would be nice is we had a way to cause OnHaltAsync to be invoked on all actors, without having to explicitly send HaltEvent to each of them - outside code may not even know all the ActorId's to do this, whereas the Runtime does. Perhaps we need a new async method like this:

    Task HaltAsync();

And the ActorRuntime can implement this by calling OnHaltAsync on all actors that are not already halted.