Move per-sitemap url counts to post metas

Moves the individual sitemap statistics into post metas and create an option for tracking the total sitemap count. This is to stop and options from getting beyond the size limit on large sites.

Also fixes related UT's and clears sitemap stats and xml data when sitemaps are reset. Lastly, adds a better reset message for that.

The reset message depends on #62 to be displayed work properly.