func NewSyncer(c *clientv3.Client, prefix string, rev int64) Syncer {
	return &syncer{c: c, prefix: prefix, rev: rev}
}
func NewSyncer(c *clientv3.Client, prefix string, rev int64) Syncer {
	return &syncer{c: c, prefix: prefix, rev: rev}
}
// NewSyncer creates a Syncer.