[SPARK-7826] [CORE] Suppress extra calling getCacheLocs.

There are too many extra call method `getCacheLocs` for `DAGScheduler`, which includes Akka communication.
To improve `DAGScheduler` performance, suppress extra calling the method.

In my application with over 1200 stages, the execution time became 3.8 min from 8.5 min with my patch.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #6352 from ueshin/issues/SPARK-7826 and squashes the following commits:

3d4d036 [Takuya UESHIN] Modify a test and the documentation.
10b1b22 [Takuya UESHIN] Simplify the unit test.
d858b59 [Takuya UESHIN] Move the storageLevel check inside the if (!cacheLocs.contains(rdd.id)) block.
6f3125c [Takuya UESHIN] Fix scalastyle.
b9c835c [Takuya UESHIN] Put the condition that checks if the RDD has uncached partition or not into variable for readability.
f87f2ec [Takuya UESHIN] Get cached locations from block manager only if the storage level of the RDD is not StorageLevel.NONE.
8248386 [Takuya UESHIN] Revert "Suppress extra calling getCacheLocs."
a4d944a [Takuya UESHIN] Add an unit test.
9a80fad [Takuya UESHIN] Suppress extra calling getCacheLocs.