Job cancellation: addressed code review feedback from Kay.

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/executor/Executor.scala

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

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

.../scala/org/apache/spark/scheduler/DAGSchedulerEvent.scala

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

...scala/org/apache/spark/scheduler/SchedulableBuilder.scala

...org/apache/spark/scheduler/cluster/ClusterScheduler.scala

...pache/spark/scheduler/cluster/ClusterTaskSetManager.scala

...ala/org/apache/spark/scheduler/local/LocalScheduler.scala

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