func NewWatcher(w clientv3.Watcher, prefix string) clientv3.Watcher {
	return &watcherPrefix{Watcher: w, pfx: prefix, stopc: make(chan struct{})}
}
func NewWatcher(w clientv3.Watcher, prefix string) clientv3.Watcher {
	return &watcherPrefix{Watcher: w, pfx: prefix, stopc: make(chan struct{})}
}
// NewWatcher wraps a Watcher instance so that all Watch requests
// are prefixed with a given string and all Watch responses have
// the prefix removed.