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