Move storage package to use Digest type

Mostly, we've made superficial changes to the storage package to start using
the Digest type. Many of the exported interface methods have been changed to
reflect this in addition to changes in the way layer uploads will be initiated.

Further work here is necessary but will come with a separate PR.

storage/layerreader.go

@@ -6,6 +6,8 @@ import (
 	"io"
 	"os"
 	"time"
+
+	"github.com/docker/docker-registry/digest"
 )
 
 // layerReadSeeker implements Layer and provides facilities for reading and
@@ -16,7 +18,7 @@ type layerReader struct {
 	brd        *bufio.Reader
 
 	name      string // repo name of this layer
-	tarSum    string
+	digest    digest.Digest
 	path      string
 	createdAt time.Time
 
@@ -35,8 +37,8 @@ func (lrs *layerReader) Name() string {
 	return lrs.name
 }
 
-func (lrs *layerReader) TarSum() string {
-	return lrs.tarSum
+func (lrs *layerReader) Digest() digest.Digest {
+	return lrs.digest
 }
 
 func (lrs *layerReader) CreatedAt() time.Time {