Once driver register successfully, stop it to connect to master.

This commit is to resolve SPARK-12396.

Author: echo2mei <534384876@qq.com>

Closes #10354 from echoTomei/master.

core/src/main/scala/org/apache/spark/deploy/client/AppClient.scala

@@ -130,6 +130,7 @@ private[spark] class AppClient(
             if (registered.get) {
               registerMasterFutures.get.foreach(_.cancel(true))
               registerMasterThreadPool.shutdownNow()
+              registrationRetryTimer.cancel(true)
             } else if (nthRetry >= REGISTRATION_RETRIES) {
               markDead("All masters are unresponsive! Giving up.")
             } else {