SPARK-1715: Ensure actor is self-contained in DAGScheduler

https://issues.apache.org/jira/browse/SPARK-1715

Though the current supervisor-child structure works fine for fault-tolerance, it violates the basic rule that the actor is better to be self-contained

We should forward the message from supervisor to the child actor, so that we can eliminate the hard-coded timeout threshold for starting the DAGScheduler and provide more convenient interface for future development like parallel DAGScheduler, or new changes to the DAGScheduler