Part of [SPARK-2456] Removed some HashMaps from DAGScheduler by storing information in Stage.

This is part of the scheduler cleanup/refactoring effort to make the scheduler code easier to maintain. 

@kayousterhout @markhamstra please take a look ... 
Yes, and you've got it after filterKeys just like after filter.  The result of filterKeys is a Map[Int, Stage], and nothing needs to change within the foreach { ... }.