Load only paths, not full mappings, for sitemaps

This avoids loading mapping objects into memory when we only need
the paths. Numbers from dev with 4GB RAM, using a single unicorn,
after 10 requests:

master: 5-5.5s request time, 8.5% RAM
here: 3.5s request time, 4% RAM
Merge pull request #120 from alphagov/pluck_paths_for_sitemap

Load only paths, not full mappings, for sitemaps