In the stop method of ConnectionManager to cancel the ackTimeoutMonitor

cc JoshRosen sarutak

Author: GuoQiang Li <witgo@qq.com>

Closes #1989 from witgo/cancel_ackTimeoutMonitor and squashes the following commits:

4a700fa [GuoQiang Li] In the stop method of ConnectionManager to cancel the ackTimeoutMonitor

core/src/main/scala/org/apache/spark/network/ConnectionManager.scala

@@ -886,6 +886,7 @@ private[spark] class ConnectionManager(
   }
 
   def stop() {
+    ackTimeoutMonitor.cancel()
     selectorThread.interrupt()
     selectorThread.join()
     selector.close()