Memorise last state

In many cases it's probably not necessary to regenerate the sitemap.. Can't we cache what we've seen (..last?) and only regenerate the sitemap if there are actual news?

In that case we could also use the ping feature of sitemap_python:
https://github.com/socrateslee/sitemap_python/tree/073c4174d42d635f4dada41bd2c040fa3a2e058c#ping-search-engine