Automatic merge from submit-queue
etcd3/store: watcher implementation
ref: https://github.com/kubernetes/kubernetes/issues/22448
This PR does:
- Provide a watcher that uses etcd v3 API to watch changes via etcd and process them based on existing logic of storage.Interface.Watch(), WatchList().
- By using the watcher, very trivial to implement Watch() and WatchList() in etcd3 storage.Interface implementation.