Pull scan query scrolling out into its own class.
Add an enumerator for all document links.

This should mean we can iterate through the links to generate sitemaps
much more quickly, because we aren't loading in the entire document
bodies from elasticsearch and we aren't taking the time to construct
Document objects.
Re-factor sitemap generation

This will allow us to write tests and provides clearer separation of concerns.
Merge branch 'link-enumerator' into sitemap
Optimise retrieval of all_documents

Using an enumerator to avoid converting to an array.
Use all_document_links which returns links only, rather than the entire document.
Only include the site root once. Good.