[CodeReview 3.0] Dubbo in spring

### 1) DubboConfigConfigurationRegistrar duplicate with DubboComponentScanRegistrar.
DubboConfigConfigurationRegistrar only has one function call:
```java
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
// initialize dubbo beans
DubboSpringInitializer.initialize(registry);
}
```
but DubboComponentScanRegistrar also has this function call:
```java
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
// initialize dubbo beans
DubboSpringInitializer.initialize(registry);

Set packagesToScan = getPackagesToScan(importingClassMetadata);

registerServiceAnnotationPostProcessor(packagesToScan, registry);
}
```