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 of hashing algorithms. As a POC, the VersionDev
Tarsum does not include the mtime in the checksum calculation, and would
solve https://github.com/docker/docker/issues/7387 though this is not a
settled Version is subject to change until a version number is assigned.

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

docs/session.go

@@ -407,7 +407,10 @@ func (r *Session) PushImageLayerRegistry(imgID string, layer io.Reader, registry
 
 	log.Debugf("[registry] Calling PUT %s", registry+"images/"+imgID+"/layer")
 
-	tarsumLayer := &tarsum.TarSum{Reader: layer}
+	tarsumLayer, err := tarsum.NewTarSum(layer, false, tarsum.Version0)
+	if err != nil {
+		return "", "", err
+	}
 	h := sha256.New()
 	h.Write(jsonRaw)
 	h.Write([]byte{'\n'})