Use CompletableFuture to rewrite the embedded ListenableFuture.

[chickenlj]

We have some functionalities which were implemented with ListenableFuture, ListenableFuture is supplementary for Future introduced in Java 5. Now I think we can rewrite those codes with CompletableFuture provided by Java 8.

• The ListenableFuture related classes are defined here: https://github.com/apache/incubator-dubbo/tree/master/dubbo-common/src/main/java/org/apache/dubbo/common/concurrent
• Classes use ListenableFuture
  • ZkClientWrapper. https://github.com/apache/incubator-dubbo/blob/master/dubbo-remoting/dubbo-remoting-zookeeper/src/main/java/org/apache/dubbo/remoting/zookeeper/zkclient/ZkClientWrapper.java
  • AbstractMonitorFactory. https://github.com/apache/incubator-dubbo/blob/master/dubbo-monitor/dubbo-monitor-api/src/main/java/org/apache/dubbo/monitor/support/AbstractMonitorFactory.java