[SPARK-6394][Core] cleanup BlockManager companion object and improve the...

[SPARK-6394][Core] cleanup BlockManager companion object and improve the getCacheLocs method in DAGScheduler

The current implementation include searching a HashMap many times, we can avoid this.
Actually if you look into `BlockManager.blockIdsToBlockManagers`, the core function call is [this](https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/storage/BlockManager.scala#L1258), so we can call `blockManagerMaster.getLocations` directly and avoid building a HashMap.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #5043 from cloud-fan/small and squashes the following commits:

e959d12 [Wenchen Fan] fix style
203c493 [Wenchen Fan] some cleanup in BlockManager companion object
d409099 [Wenchen Fan] address rxin's comment
faec999 [Wenchen Fan] add regression test
2fb57aa [Wenchen Fan] imporve the getCacheLocs method