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 ...

Author: Reynold Xin <rxin@apache.org>

Closes #1561 from rxin/dagSchedulerHashMaps and squashes the following commits:

1c44e15 [Reynold Xin] Clear pending tasks in submitMissingTasks.
620a0d1 [Reynold Xin] Use filterKeys.
5b54404 [Reynold Xin] Code review feedback.
c1e9a1c [Reynold Xin] Removed some HashMaps from DAGScheduler by storing information in Stage.
[SPARK-2659][SQL] Fix division semantics for hive

Author: Michael Armbrust <michael@databricks.com>

Closes #1557 from marmbrus/fixDivision and squashes the following commits:

b85077f [Michael Armbrust] Fix unit tests.
af98f29 [Michael Armbrust] Change DIV to long type
0c29ae8 [Michael Armbrust] Fix division semantics for hive