Refer previously indexed documents to updating/deleting child documents

In #58, as @pyromaniac suggested:

> So I'm thinking about in case of parent mapping presence - just get all the reindexed documents from ES with scroll API and compare parent ids with those I'm going to index. If there is no parent_id (in case of deletion by id) then just add it. If parent_id was changed - then split this document-related bulk query part into two stages: delete and then index.

I've added `Chewy::Type::Import::fetch_indexed_objects` which fetches indexed documents having IDs of objects to index (using ids query), and refer the objects when building body for bulk request.

Thanks!
What if parent is not used for this type at all?