[SPARK-4282][YARN] Stopping flag in YarnClientSchedulerBackend should be volatile

In YarnClientSchedulerBackend, a variable "stopping" is used as a flag and it's accessed by some threads so it should be volatile.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3143 from sarutak/stopping-flag-volatile and squashes the following commits:

58fdcc9 [Kousuke Saruta] Marked stoppig flag as volatile

yarn/common/src/main/scala/org/apache/spark/scheduler/cluster/YarnClientSchedulerBackend.scala

@@ -33,7 +33,7 @@ private[spark] class YarnClientSchedulerBackend(
 
   private var client: Client = null
   private var appId: ApplicationId = null
-  private var stopping: Boolean = false
+  @volatile private var stopping: Boolean = false
 
   /**
    * Create a Yarn client to submit an application to the ResourceManager.