Prevent ContextClassLoader of Actor from becoming ClassLoader of Executor

Constructor of org.apache.spark.executor.Executor should not set context class loader of current thread, which is backend Actor's thread.

Run the following code in local-mode REPL.

scala> case class Foo(i: Int)
scala> val ret = sc.parallelize((1 to 100).map(Foo), 10).collect

This causes errors as follows:

ERROR actor.OneForOneStrategy: [L$line5.$read$$iwC$$iwC$$iwC$$iwC$Foo;
java.lang.ArrayStoreException: [L$line5.$read$$iwC$$iwC$$iwC$$iwC$Foo;
at scala.runtime.ScalaRunTime$.array_update(ScalaRunTime.scala:88)
at org.apache.spark.SparkContext$$anonfun$runJob$3.apply(SparkContext.scala:870)
at org.apache.spark.SparkContext$$anonfun$runJob$3.apply(SparkContext.scala:870)
at org.apache.spark.scheduler.JobWaiter.taskSucceeded(JobWaiter.scala:56)
at org.apache.spark.scheduler.DAGScheduler.handleTaskCompletion(DAGScheduler.scala:859)
at org.apache.spark.scheduler.DAGScheduler.processEvent(DAGScheduler.scala:616)
at org.apache.spark.scheduler.DAGScheduler$$anonfun$start$1$$anon$2$$anonfun$receive$1.applyOrElse(DAGScheduler.scala:207)
at akka.actor.ActorCell.receiveMessage(ActorCell.scala:498)
at akka.actor.ActorCell.invoke(ActorCell.scala:456)
at akka.dispatch.Mailbox.processMailbox(Mailbox.scala:237)
at akka.dispatch.Mailbox.run(Mailbox.scala:219)
at akka.dispatch.ForkJoinExecutorConfigurator$AkkaForkJoinTask.exec(AbstractDispatcher.scala:386)
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:260)
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1339)
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1979)
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:107)

This is because the class loaders to deserialize result Foo instances might be different from backend Actor's, and the Actor's class loader should be the same as Driver's.

See PR: https://github.com/apache/spark/pull/15