SPARK-1235: manage the DAGScheduler EventProcessActor with supervisor and refactor the DAGScheduler with Akka

https://spark-project.atlassian.net/browse/SPARK-1235

In the current implementation, the running job will hang if the DAGScheduler crashes for some reason (eventProcessActor throws exception in receive() )

The reason is that the actor will automatically restart when the exception is thrown during the running but is not captured properly (Akka behaviour), and the JobWaiters are still waiting there for the completion of the tasks

In this patch, I refactored the DAGScheduler with Akka and manage the eventProcessActor with supervisor, so that upon the failure of a eventProcessActor, the supervisor will terminate the EventProcessActor and close the SparkContext 

thanks for @kayousterhout and @markhamstra to give the hints in JIRA