Add lastmod times for individual sitemaps in the sitemap index, to hopefully help search engines keep up in the case of unchanged files.
Support using existing $map object for hash_sitemap, rather than reloading from disk.
Merge branch 'sitemaps-tweaks'

* sitemaps-tweaks:
  Support using existing $map object for hash_sitemap, rather than reloading from disk.
  Add lastmod times for individual sitemaps in the sitemap index, to hopefully help search engines keep up in the case of unchanged files.