Rethink full sitemap generation to allow parallel processing

[pkevan]

Currently, the full sitemap generator uses option values to store the sequence of initial sitemap to generate, which can cause issues on sites where parallel processing is possible.

Rethinking this method to use something which reduces a potential race condition and allows them to be generated quicker would be good - perhaps even dropping the individual cron event setup would be better here.

[mjangda]

Maybe we should just use a custom table to build and track the queue of dates to be processed?

[pkevan]

Yes, exactly or some kind of postmeta could also work.