Whiteout portability

Continuity has the potential to be used to store filesystem "diffs" in the same way tar files are used in Docker today. In order to support these diffs "whiteout" files should have first class support in the manifest. This will allow a portable way to use whiteouts between different copy on write file systems despite where it was originally generated. As a research item, how are whiteout files handled in the tar file diffs used by docker and are there any issues across filesystems (i.e export from AUFS import to overlay).