- [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/)
}
            final int defaultPort = ExtensionLoader.getExtensionLoader(Protocol.class).getExtension(name).getDefaultPort();
            if (portToBind == null || portToBind == 0) {
                portToBind = defaultPort;
            }
            if (portToBind <= 0) {
                portToBind = getRandomPort(name);
                if (portToBind == null || portToBind < 0) {
                    portToBind = getAvailablePort(defaultPort);
                    putRandomPort(name, portToBind);
                }
            }
        }
        // 
        map.put(Constants.BIND_PORT_KEY, String.valueOf(portToBind));
        // 
        String portToRegistryStr = getValueFromConfig(protocolConfig, DUBBO_PORT_TO_REGISTRY);
        Integer portToRegistry = parsePort(portToRegistryStr);
        if (portToRegistry == null) {
            portToRegistry = portToBind;
        }
        return portToRegistry;
    }
    private Integer parsePort(String configPort) {
        Integer port = null;
        if (configPort != null && configPort.length() > 0) {
            try {
                Integer intPort = Integer.parseInt(configPort);
                if (isInvalidPort(intPort)) {
                    throw new IllegalArgumentException("Specified invalid port from env value:" + configPort);