Cache registered autoloaders

This saves more than 20ms (!) on every request, the previous problem was that `\OC_App::registerAutoloading` calls `\OC\AppFramework\App::buildAppNamespace` which parses the appinfo.xml. Since that was also called multiple times (e.g. on cloud.nextcloud.com over 200 times) that had a significant performance impact. Also on simple PROPFIND requests.

https://blackfire.io/profiles/compare/65a53e6e-7f35-4974-b559-4c81abd01c3b/graph shows the difference nicely.