iterator.go
func (it *Iterator) Valid() bool {
	if it.item == nil {
		return false
	}
	return bytes.HasPrefix(it.item.key, it.opt.Prefix)
}
func (it *Iterator) Valid() bool {
	if it.item == nil {
		return false
	}
	return bytes.HasPrefix(it.item.key, it.opt.Prefix)
}
// Valid returns false when iteration is done.