[SPARK-3567] appId field in SparkDeploySchedulerBackend should be volatile

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

Closes #2428 from sarutak/appid-volatile-modification and squashes the following commits:

c7d890d [Kousuke Saruta] Added volatile modifier to appId field in SparkDeploySchedulerBackend

core/src/main/scala/org/apache/spark/scheduler/cluster/SparkDeploySchedulerBackend.scala

@@ -34,7 +34,7 @@ private[spark] class SparkDeploySchedulerBackend(
   var client: AppClient = null
   var stopping = false
   var shutdownCallback : (SparkDeploySchedulerBackend) => Unit = _
-  var appId: String = _
+  @volatile var appId: String = _
 
   val registrationLock = new Object()
   var registrationDone = false