Best practice for "double" pagination

## New issue checklist

- [x] I have reviewed the [`README`](https://github.com/Instagram/IGListKit/blob/master/README.md) and [documentation](http://instagram.github.io/IGListKit)
- [x] I have searched [existing issues](https://github.com/Instagram/IGListKit/issues) and this is not a duplicate

Hello everyone!

We have a list that displays photos grouped by album. Albums and their photos are being retrieved by two different GraphQL queries that return results in a paginated manner. We're planning to use a `ListStackedSectionController` for each album-and-its-photos item with one section ctrl for the "album header cell" and one section controller to manage all of it's photos.

Wanted to ask if it makes sense to have the pages fetching logic (keeping offsets and issuing network requests for next page of photos) inside the section ctrl that manages the album's photos?