While attempting to fix#2782 I thought the problem was the
CheckFolderHealth method, so I cleaned it up. That turned out not to be
the case, but I think this is better anyhow.
It also moves the "create folder and marker if the folder was empty in
the index" code to StartFolder where I think it makes better sense.
This is covered by a number of existing tests.
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3343