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>