stageId <--> jobId mapping in DAGScheduler

Okay, I think this one is ready to go -- or at least it's ready for review and discussion.  It's a carry-over of https://github.com/mesos/spark/pull/842 with updates for the newer job cancellation functionality.  The prior discussion still applies.  I've actually changed the job cancellation flow a bit: Instead of ``cancelTasks`` going to the TaskScheduler and then ``taskSetFailed`` coming back to the DAGScheduler (resulting in ``abortStage`` there), the DAGScheduler now takes care of figuring out which stages should be cancelled, tells the TaskScheduler to cancel tasks for those stages, then does the cleanup within the DAGScheduler directly without the need for any further prompting by the TaskScheduler.

I know of three outstanding issues, each of which can and should, I believe, be handled in follow-up pull requests:

1) https://spark-project.atlassian.net/browse/SPARK-960
2) JobLogger should be re-factored to eliminate duplication
3) Related to 2), the WebUI should also become a consumer of the DAGScheduler's new understanding of the relationship between jobs and stages so that it can display progress indication and the like grouped by job.  Right now, some of this information is just being sent out as part of ``SparkListenerJobStart`` messages, but more or different job <--> stage information may need to be exported from the DAGScheduler to meet listeners needs.

d0d7a4f can be cherry-picked almost cleanly into branch-0.8.  A little merging is needed in MapOutputTracker and the DAGScheduler.  Merged versions of those files are in https://github.com/markhamstra/incubator-spark/tree/512c8e54aefaf534914b11812370c9de46a5c3fc