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.
Merge branch 'master' into beta