Fix file cache updater

This fixes bug https://github.com/owncloud/core/issues/5098
For a detailed explanation how to reproduce it look at: https://github.com/owncloud/mirall/issues/1056#issuecomment-25524490

@icewind1991 please have a look. correctFolder() always assumed that we are operating on a folder owned by the current user which of course is wrong in case of a shared file. That's why I changed resolvePath() to resolve the path to the owners path and updated the correctFolder() method to loop though the correct folder tree.