[SPARK-4655][Core] Split Stage into ShuffleMapStage and ResultStage subclasses

Hi all - this patch changes the Stage class to an abstract class and introduces two new classes that extend it: ShuffleMapStage and ResultStage - with the goal of increasing readability of the DAGScheduler class. Their usage is updated within DAGScheduler.

Author: Ilya Ganelin <ilya.ganelin@capitalone.com>
Author: Ilya Ganelin <ilganeli@gmail.com>

Closes #4708 from ilganeli/SPARK-4655 and squashes the following commits:

c248924 [Ilya Ganelin] Merge branch 'SPARK-4655' of github.com:ilganeli/spark into SPARK-4655
d930385 [Ilya Ganelin] Fixed merge conflict from
a9a765f [Ilya Ganelin] Update DAGScheduler.scala
c03563c [Ilya Ganelin] Minor fixeS
c39e971 [Ilya Ganelin] Added return typing for public methods
845bc87 [Ilya Ganelin] Merge branch 'SPARK-4655' of github.com:ilganeli/spark into SPARK-4655
e8031d8 [Ilya Ganelin] Minor string fixes
4ec53ac [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-4655
c004f62 [Ilya Ganelin] Update DAGScheduler.scala
a2cb03f [Ilya Ganelin] [SPARK-4655] Replaced usages of Nil and eliminated some code reuse
3d5cf20 [Ilya Ganelin] [SPARK-4655] Moved mima exclude to 1.4
6912c55 [Ilya Ganelin] Resolved merge conflict
4bff208 [Ilya Ganelin] Minor stylistic fixes
c6fffbb [Ilya Ganelin] newline
41402ad [Ilya Ganelin] Style fixes
02c6981 [Ilya Ganelin] Merge branch 'SPARK-4655' of github.com:ilganeli/spark into SPARK-4655
c755a09 [Ilya Ganelin] Some more stylistic updates and minor refactoring
b6257a0 [Ilya Ganelin] Update MimaExcludes.scala
0f0c624 [Ilya Ganelin] Fixed merge conflict
2eba262 [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-4655
6b43d7b [Ilya Ganelin] Got rid of some spaces
6f1a5db [Ilya Ganelin] Revert "More minor formatting and refactoring"
1b3471b [Ilya Ganelin] Merge remote-tracking branch 'upstream/master' into SPARK-4655
c9288e2 [Ilya Ganelin] More minor formatting and refactoring
d548caf [Ilya Ganelin] Formatting fix
c3ae5c2 [Ilya Ganelin] Explicit typing
0dacaf3 [Ilya Ganelin] Got rid of stale import
6da3a71 [Ilya Ganelin] Trailing whitespace
b85c5fe [Ilya Ganelin] Added minor fixes
a57dfcd [Ilya Ganelin] Added MiMA exclusion to get around binary compatibility check
83ed849 [Ilya Ganelin] moved braces for consistency
96dd161 [Ilya Ganelin] Fixed minor style error
cfd6f10 [Ilya Ganelin] Updated DAGScheduler to use new ResultStage and ShuffleMapStage classes
83494e9 [Ilya Ganelin] Added new Stage classes