Optimize the logic in org.apache.dubbo.common.URL#getEncodedServiceKey()

The currently implementation of `org.apache.dubbo.common.URL#getEncodedServiceKey()` is not elegant.

```java
public String getEncodedServiceKey() {
String serviceKey = this.getServiceKey();
serviceKey = serviceKey.replaceFirst("/", "*");
return serviceKey;
}
```

The purpose of replacing "/" with "*" is that Zookeeper does not support "/" as a valid character of key.

Proposal:

Add new parameter to `getServiceKey()`, for example: `getServiceKey(String delimiter)`, and pass "/" or "*" in different scenario.