Switching to in-memory-shuffle based on changes described in https://github.com/amplab/graphx/commits/sigmod-memory-shuffle

Conflicts:
	core/src/main/scala/org/apache/spark/scheduler/ShuffleMapTask.scala
	core/src/main/scala/org/apache/spark/storage/BlockManager.scala
	core/src/main/scala/org/apache/spark/storage/BlockObjectWriter.scala
	graphx/src/main/scala/org/apache/spark/graphx/Pregel.scala
using a foreach instead of mapPartitions