Added stageId <--> jobId mapping in DAGScheduler
  ...and make sure that DAGScheduler data structures are cleaned up on job completion.
  Initial effort and discussion at https://github.com/mesos/spark/pull/842

Conflicts:
	core/src/main/scala/org/apache/spark/MapOutputTracker.scala
        core/src/main/scala/org/apache/spark/scheduler/DAGScheduler.scala