Update storage package to use StorageDriver.Stat

This change updates the backend storage package that consumes StorageDriver to
use the new Stat call, over CurrentSize. It also makes minor updates for using
WriteStream and ReadStream.

storage/layerstore.go

@@ -1,8 +1,6 @@
 package storage
 
 import (
-	"time"
-
 	"github.com/docker/docker-registry/digest"
 	"github.com/docker/docker-registry/storagedriver"
 )
@@ -55,11 +53,6 @@ func (ls *layerStore) Fetch(name string, digest digest.Digest) (Layer, error) {
 		fileReader: *fr,
 		name:       name,
 		digest:     digest,
-
-		// TODO(stevvooe): Storage backend does not support modification time
-		// queries yet. Layers "never" change, so just return the zero value
-		// plus a nano-second.
-		createdAt: (time.Time{}).Add(time.Nanosecond),
 	}, nil
 }