func (i *InmemStore) LastIndex() (uint64, error) {
	i.l.RLock()
	defer i.l.RUnlock()
	return i.highIndex, nil
}
func (i *InmemStore) LastIndex() (uint64, error) {
	i.l.RLock()
	defer i.l.RUnlock()
	return i.highIndex, nil
}
// LastIndex implements the LogStore interface.