FsTraversal now provided stat structures

The `FsTraversal` template needs to call `stat()` on each directory entry it processes. Until now these `struct stat` information was lost and the users of `FsTraversal` needed to do the `stat()` call again.
This Pull Request provides the user with the `struct stat` obtained while traversing a directory structure. It contains quite some changes, but most of them account for API adaption of the new callback format.

**Note:** This optimization will be used in the new file processing backend.