clientv3/concurrency/mutex.go
func NewLocker(s *Session, pfx string) sync.Locker {
	return &lockerMutex{NewMutex(s, pfx)}
}
func NewLocker(s *Session, pfx string) sync.Locker {
	return &lockerMutex{NewMutex(s, pfx)}
}
// NewLocker creates a sync.Locker backed by an etcd mutex.