raft/storage.go
func (ms *MemoryStorage) LastIndex() (uint64, error) {
	ms.Lock()
	defer ms.Unlock()
	return ms.lastIndex(), nil
}
func (ms *MemoryStorage) LastIndex() (uint64, error) {
	ms.Lock()
	defer ms.Unlock()
	return ms.lastIndex(), nil
}
// LastIndex implements the Storage interface.