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

### Environment

* Dubbo version: 2.6.2
* Operating System version: Linux
* Java version: 1.8

### Steps to reproduce this issue

1. Restart dubbo service.
2. Ping hostname becomes slow.


### Expected Result

Start dubbo service quickly.

### Actual Result

![image](https://user-images.githubusercontent.com/15684156/61106851-f4d04500-a4b0-11e9-898a-4c55a78f7654.png)

Dubbo service startup is slow, it takes two hours.

Thread dump like this:

![image](https://user-images.githubusercontent.com/15684156/61106974-4678cf80-a4b1-11e9-9c03-67a9f39f5d99.png)

[More details](https://crossoverjie.top/2019/07/05/troubleshoot/dubbo-start-slow/)
public boolean isUnexported() {
        return unexported;
    }
    public void checkAndUpdateSubConfigs() {
        // 
        completeCompoundConfigs();
        // 
        startConfigCenter();
        checkDefault();
        checkProtocol();
        checkApplication();
        // 
        if (!isOnlyInJvm()) {
            checkRegistry();
        }
        this.refresh();
        checkMetadataReport();
        if (StringUtils.isEmpty(interfaceName)) {
            throw new IllegalStateException("<dubbo:service interface=\"\" /> interface not allow null!");
        }
        if (ref instanceof GenericService) {
            interfaceClass = GenericService.class;
            if (StringUtils.isEmpty(generic)) {
                generic = Boolean.TRUE.toString();
            }
        } else {
            try {
                interfaceClass = Class.forName(interfaceName, true, Thread.currentThread()
                        .getContextClassLoader());
            } catch (ClassNotFoundException e) {