While the sitemaps are redundant and those may exceed the 50 MB limit in the future if we don't split them, the `sitemap_urls` data itself is a very small chunk. [Here's an output](https://bug906882.bugzilla.mozilla.org/attachment.cgi?id=8335803). It's currently only 12 KB. I think it will never exceed the 1 MB memcache limit.