[Discussion]Java Chassis Support Gracefully Shutdown

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Hi, All:
   We did not implement gracefully shutdown yet: 
  
           // ??????????????????????????????????kill -9 {pid}??????
          // 1. ??????????????
          // TODO ????????????
          if (applicationContext instanceof AbstractApplicationContext) {
            ((AbstractApplicationContext) 
applicationContext).registerShutdownHook();
          }
  
   And had got an issue : 
https://github.com/apache/incubator-servicecomb-java-chassis/issues/685 from 
user.
   So i think it's time we must do it and remove this TODO ?
  
 Best Regards!
 YangYongZheng