Collections with backfill

This isn't a serious pull request at this point -- it's not complete; I need Content API to be returning the backfill query string first -- but rather a request for comment.

A basic requirement anyone calling the `collection` endpoint will have is to call the backfill query immediately afterwards with all of the same query parameters (in terms of what fields they want to show). It would make sense to me if the client implemented this functionality, so I've sketched out how that might be achieved below.

As I imagine there may be disagreement about whether this belongs in the Content API client this pull request is a chance for anyone to register that now (rather than after I've done the main brunt of the work).