Fixes: U4-5055 Umbraco update fails with large set of media content due to timeout in RebuildXmlStructures. Adds RebuildXmlStructures to content, media, member services as public APIs (though people won't really use them), the underlying repositories now rebuild these structures using a single transactions but queried by pages of 5000 which should reduce memory overhead if there's a ton of media, etc... Added tests for this too. Added CountPublished to ContentService too.