snapshot: add Close()

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

metadata/snapshot.go

@@ -724,3 +724,8 @@ func (s *snapshotter) pruneBranch(ctx context.Context, node *treeNode) error {
 
 	return nil
 }
+
+// Close closes s.Snapshotter but not db
+func (s *snapshotter) Close() error {
+	return s.Snapshotter.Close()
+}