Added stageId <--> jobId mapping in DAGScheduler

I'm not completely certain that this is ready to be merged yet, but I think it is ready to gather some comments.

Outside of DAGScheduler.scala, the changes are almost entirely just renaming -- I resolved the confusion of the notions of priority and job identity by consistently relabeling old, FIFO-only-based usages of "priority" to now be "jobId".  The old usage of "priority" was misleading since we now have the fair scheduler infrastructure that doesn't use the old "priority" to order priority except to break what would otherwise be priority ties.  Now we can speak consistently about jobs having jobIds that are unique and always increasing.  Stages contain the jobId rof the job which was responsible for creating them.  Again, all of that is really just renaming or thinking about things slightly differently without changing anything functionally.

There are also a few easy changes within DAGScheduler.scala that just suppress a bunch of code analyzer warnings in IntelliJ -- I got tired of looking at them.

The bigger changes in the DAGScheduler have to do with the new jobIdToStageIds and stageIdToJobIds maps and the associated methods to build them up and tear them down.  (I also renamed idToStage to stageIdToStage to be more consistent and less ambiguous.)  The stageIdToJobIds map gets built up as new stages are created, the jobIdToStageIds map gets sync'ed-with/built-up from stageIdToJobIds when jobs are submitted, and both maps are modified in concert when jobs are completed or aborted.

There's a fair amount of walking through entire data structures to create and maintain these maps, so I was concerned about the performance hit.  I haven't rigorously tested just the relative performance of the DAGScheduler in isolation with and without these changes, but I have closely watched the total time reported by a complete run of Spark's unit tests, and there appears to be no significant difference -- a complete run takes almost 13.5 minutes when my machine is fully warmed up, and the difference with the new stageId <--> jobId mapping is only a couple of seconds.  Some of that is probably because, while a lot more is done when a new stage is created, that actually results in the short-circuiting of later calls to, e.g., getShuffleMapStage.  

The last bits outside the DAGScheduler have to do with SparkListenerJobStart, which now sends along an array containing all of the stageIds that the job depends on.

With all of this in place, it should now be simple for the WebUI to aggregate the appropriate stages to report progress and other information for an entire job, and it should also be easy for Ram's killJob stuff to kill all of the stages associated with a job while not killing any that are also associated with another job.

From looking at some instrumented output, I'm fairly certain that I've got things at least close to right.  I don't think this is quite ready to merge yet, since the JobLogger still probably needs to be tweaked, and I need to come to an understanding of why stages seem to be left alive by the ThreadingSuite.  Outside of that, this new code seems to do a nice job preventing the sizes of various DAGScheduler data structures from growing without bound, and does so while not breaking any of the unit tests.