func (s *MergeIterator) Key() []byte {
	if len(s.h) == 0 {
		return nil
	}
	return s.h[0].itr.Key()
}
func (s *MergeIterator) Key() []byte {
	if len(s.h) == 0 {
		return nil
	}
	return s.h[0].itr.Key()
}
// Key returns the key associated with the current iterator