回调管理,处理当grpc加入或者删除时候的回调
===== WatchCallBackMgr =====
→(a)**list:map[int32][]func(string,string)**\\
→→回调map

→(a)**watchEventMutex sync.RWMutex**\\
→→map锁

→(f)**addWatchCallbacks(eventType:int32,callback:func(etcdPath:string,addr:string))**\\
→→添加回调，eventType是类型，比如：clientv3.EventTypePut或者clientv3.EventTypeDelete。callback就是具体的方法咯

→(f)**invokeWatchCallBacks(eventType int32,etcdPath:string,addr:string)**\\
→→执行回调
===== Instances =====
单例\\
→(a)**watchCallbackMgr:WatchCallbackMgr**