inmem_store.go
func (i *InmemStore) FirstIndex() (uint64, error) {
	i.l.RLock()
	defer i.l.RUnlock()
	return i.lowIndex, nil
}
func (i *InmemStore) FirstIndex() (uint64, error) {
	i.l.RLock()
	defer i.l.RUnlock()
	return i.lowIndex, nil
}
// FirstIndex implements the LogStore interface.