code: public void notifyStateChangeToAvailable() {
          final List<ConsumerStateListener> onAvailable = consumerConfig.getOnAvailable();
          if (onAvailable != null) {
              AsyncRuntime.getAsyncThreadPool().execute(new Runnable() {
                  @Override
                  public void run() {
                      // 状态变化通知监听器
                      for (ConsumerStateListener listener : onAvailable) {
                          try {
                              listener.onAvailable(consumerConfig.getConsumerBootstrap().getProxyIns());
                          } catch (Exception e) {
                              LOGGER.warnWithApp(consumerConfig.getAppName(),
                                  "Failed to notify consumer state listener when state change to available");
                          }
                      }
                  }
              });
          }
      }
docs: 通知状态变成可用,主要是：<br>
  1.启动成功变成可用时<br>
  2.注册中心增加，更新节点后变成可用时<br>
  3.重连上从一个可用节点都没有变成有可用节点时