- [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();
        }
      ...
    }
```

import org.apache.dubbo.rpc.RpcException;
import org.apache.dubbo.rpc.model.ApplicationModel;
import org.apache.dubbo.rpc.protocol.AbstractProxyProtocol;
import org.apache.http.HeaderElement;
import org.apache.http.HeaderElementIterator;
import org.apache.http.HttpResponse;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.config.SocketConfig;
import org.apache.http.conn.ConnectionKeepAliveStrategy;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;
import org.apache.http.message.BasicHeaderElementIterator;
import org.apache.http.protocol.HTTP;
import org.apache.http.protocol.HttpContext;
import org.jboss.resteasy.client.jaxrs.ResteasyClient;
import org.jboss.resteasy.client.jaxrs.ResteasyClientBuilder;
import org.jboss.resteasy.client.jaxrs.ResteasyWebTarget;
import org.jboss.resteasy.client.jaxrs.engines.ApacheHttpClient4Engine;
import org.jboss.resteasy.util.GetRestful;
import javax.servlet.ServletContext;
import javax.ws.rs.ProcessingException;
import javax.ws.rs.WebApplicationException;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.TimeUnit;
public class RestProtocol extends AbstractProxyProtocol {