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 ... 
Ok don't worry about it, as discussed.   I want to remove this pendingTasks anyway, which is redundant with information stored by the TSM