-Added missing PlayerEliminatedEvent update method to GameTracker
listener interface
-Added clean method to Session API
-Made clean in World kill all robots since they were being left behind
-Made robot die set interrupted so that it completes ASAP
-Moved responsibility of cleaning a player after all their robots died,
from Robot to Session

-Added unit test for world's clean
-Added unit test for robot's clean
-When reloading a session, clean old session first