Multiple bug fixes to pass the testsuites ShuffleSuite and BlockManagerSuite.

core/src/main/scala/spark/SparkContext.scala

core/src/main/scala/spark/network/ConnectionManager.scala

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

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

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

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

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

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

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

core/src/test/scala/spark/ShuffleSuite.scala

core/src/test/scala/spark/storage/BlockManagerSuite.scala