Bugfix: SPARK-965 & SPARK-966

SPARK-965: https://spark-project.atlassian.net/browse/SPARK-965
SPARK-966: https://spark-project.atlassian.net/browse/SPARK-966

* Add back `DAGScheduler.start()`, `eventProcessActor` is created and started here.

  Notice that function is only called by `SparkContext`.

* Cancel the scheduled stage resubmission task when stopping `eventProcessActor`

* Add a new `DAGSchedulerEvent` `ResubmitFailedStages`

  This event message is sent by the scheduled stage resubmission task to `eventProcessActor`.  In this way, `DAGScheduler.resubmitFailedStages()` is guaranteed to be executed from the same thread that runs `DAGScheduler.processEvent()`.

  Please refer to discussion in [SPARK-966](https://spark-project.atlassian.net/browse/SPARK-966) for details.