Updated according to Matei's code review comment.

core/src/main/scala/spark/ShuffleFetcher.scala

core/src/main/scala/spark/SparkEnv.scala

core/src/main/scala/spark/rdd/CoGroupedRDD.scala

core/src/main/scala/spark/rdd/ShuffledRDD.scala

core/src/main/scala/spark/rdd/SubtractedRDD.scala

core/src/main/scala/spark/scheduler/ShuffleMapTask.scala

core/src/main/scala/spark/serializer/Serializer.scala

core/src/main/scala/spark/serializer/SerializerManager.scala

core/src/main/scala/spark/storage/DiskStore.scala

core/src/main/scala/spark/storage/ShuffleBlockManager.scala