merging upstream changes

README.md

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

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

.../main/scala/org/apache/spark/deploy/SparkHadoopUtil.scala

core/src/main/scala/org/apache/spark/rdd/CheckpointRDD.scala

core/src/main/scala/org/apache/spark/rdd/HadoopRDD.scala

...in/scala/org/apache/spark/scheduler/InputFormatInfo.scala

...ain/scala/org/apache/spark/scheduler/ShuffleMapTask.scala

...rg/apache/spark/scheduler/local/LocalTaskSetManager.scala

...rc/main/scala/org/apache/spark/storage/BlockManager.scala

...in/scala/org/apache/spark/storage/BlockObjectWriter.scala

...ain/scala/org/apache/spark/storage/DiskBlockManager.scala

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

.../scala/org/apache/spark/storage/ShuffleBlockManager.scala

...in/scala/org/apache/spark/storage/StoragePerfTester.scala

...rc/main/scala/org/apache/spark/util/MetadataCleaner.scala

core/src/main/scala/org/apache/spark/util/Utils.scala

.../main/scala/org/apache/spark/util/collection/BitSet.scala

.../scala/org/apache/spark/util/collection/OpenHashMap.scala

.../scala/org/apache/spark/util/collection/OpenHashSet.scala