- [x] I have searched the [issues](https://github.com/apache/incubator-dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/incubator-dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.6.5


### Confusion

the code below  create a ```connectionMonitor``` instance, but ```Start``` method seems not called
```
@Override
    protected <T> T doRefer(Class<T> serviceType, URL url) throws RpcException {
        if (connectionMonitor == null) {
            connectionMonitor = new ConnectionMonitor();
        }
      ...
    }
```

private static final int DEFAULT_PORT = 80;
    private static final String DEFAULT_SERVER = "jetty";
    private static final int HTTPCLIENTCONNECTIONMANAGER_MAXPERROUTE = 20;
    private static final int HTTPCLIENTCONNECTIONMANAGER_MAXTOTAL = 20;
    private static final int HTTPCLIENT_KEEPALIVEDURATION = 30*1000;
    private static final int HTTPCLIENTCONNECTIONMANAGER_CLOSEWAITTIME_MS = 1000;
    private static final int HTTPCLIENTCONNECTIONMANAGER_CLOSEIDLETIME_S = 30;
    private final Map<String, RestServer> servers = new ConcurrentHashMap<String, RestServer>();
    private final RestServerFactory serverFactory = new RestServerFactory();