[SPARK-10911] Executors should System.exit on clean shutdown.

Call system.exit explicitly to make sure non-daemon user threads terminate. Without this, user applications might live forever if the cluster manager does not appropriately kill them. E.g., YARN had this bug: HADOOP-12441.

Author: zhuol <zhuol@yahoo-inc.com>

Closes #9946 from zhuoliu/10911.

core/src/main/scala/org/apache/spark/executor/CoarseGrainedExecutorBackend.scala

@@ -241,6 +241,7 @@ private[spark] object CoarseGrainedExecutorBackend extends Logging {
     }
 
     run(driverUrl, executorId, hostname, cores, appId, workerUrl, userClassPath)
+    System.exit(0)
   }
 
   private def printUsageAndExit() = {