ActorPing in connection with CometActors can lead to memory retention issues. Basically, the ActorPing doesn't know to stop pinging the CometActor and even though the session is gone, the Actor lives on.

I've made two changes:
- CometActors will stop responding to messages 20 seconds after they're shut down.
- Added a running flag to the class to allow the app to stop the ActorPing thing by its own logic