Finalize TarSum Version 1 w/ refactor

The current Dev version of TurSum includes hashing of extended
file attributes and omits inclusion of modified time headers.

I refactored the logic around the version differences to make it
more clear that the difference between versions is in how tar
headers are selected and ordered.

TarSum Version 1 is now declared with the new Dev version continuing
to track it.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Not sure we still want this to be true, may be preferable to pin dev at v1 since we now have "tarsum.dev" in v2 image manifests.