repository: optimize MasterIndex.Each

Sending data through a channel at very high frequency is extremely
inefficient. Thus use simple callbacks instead of channels.

> name                old time/op  new time/op  delta
> MasterIndexEach-16   6.68s ±24%   0.96s ± 2%  -85.64%  (p=0.008 n=5+5)

internal/repository/index_test.go

@@ -355,11 +355,11 @@ func TestIndexUnserialize(t *testing.T) {
 }
 
 func listPack(idx *repository.Index, id restic.ID) (pbs []restic.PackedBlob) {
-	for pb := range idx.Each(context.TODO()) {
+	idx.Each(context.TODO(), func(pb restic.PackedBlob) {
 		if pb.PackID.Equal(id) {
 			pbs = append(pbs, pb)
 		}
-	}
+	})
 	return pbs
 }