Job cancellation: address Matei's code review feedback.

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

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

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

...cala/org/apache/spark/executor/MesosExecutorBackend.scala

...src/main/scala/org/apache/spark/rdd/AsyncRDDActions.scala

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

...rc/main/scala/org/apache/spark/rdd/InterruptibleRDD.scala

...la/org/apache/spark/rdd/MapPartitionsWithContextRDD.scala

...rc/main/scala/org/apache/spark/rdd/PairRDDFunctions.scala

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

.../main/scala/org/apache/spark/scheduler/DAGScheduler.scala

...rc/main/scala/org/apache/spark/scheduler/ResultTask.scala

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

core/src/main/scala/org/apache/spark/scheduler/Task.scala

core/src/test/scala/org/apache/spark/CheckpointSuite.scala

...rc/test/scala/org/apache/spark/JobCancellationSuite.scala

...est/scala/org/apache/spark/rdd/AsyncRDDActionsSuite.scala