func (s *MergeIterator) Seek(key []byte) {
	for _, itr := range s.all {
		itr.Seek(key)
	}
	s.initHeap()
}
func (s *MergeIterator) Seek(key []byte) {
	for _, itr := range s.all {
		itr.Seek(key)
	}
	s.initHeap()
}
// Seek brings us to element with key >= given key.