Use latest schema when sweeping entries

Default file mtime comparator does not seem to work correctly in our scenario. Some trivial ENUM extensions caused sweeping to break when two files for the same day possessed different schemas.

So instead of comparing modification times just fetch the latest schema. This should fix https://trello.com/c/TcDH9GuY/101-fix-morphlines-with-enum-user-click-list-item-buyer-want-item-list-show-item. Tested locally.

@ljank