TarSum: versioning

This introduces Versions for TarSum checksums.
Fixes: https://github.com/docker/docker/issues/7526

It preserves current functionality and abstracts the interface for
future flexibility. As a POC, the Version1 Tarsum does not include the
mtime in the checksum calculation, and would solve
https://github.com/docker/docker/issues/7387

Signed-off-by: Vincent Batts <vbatts@redhat.com>