backend: Improve Save()

As mentioned in issue [#1560](https://github.com/restic/restic/pull/1560#issuecomment-364689346)
this changes the signature for `backend.Save()`. It now takes a
parameter of interface type `RewindReader`, so that the backend
implementations or our `RetryBackend` middleware can reset the reader to
the beginning and then retry an upload operation.

The `RewindReader` interface also provides a `Length()` method, which is
used in the backend to get the size of the data to be saved. This
removes several ugly hacks we had to do to pull the size back out of the
`io.Reader` passed to `Save()` before. In the `s3` and `rest` backend
this is actively used.

internal/checker/checker_test.go

@@ -3,7 +3,9 @@ package checker_test
 import (
 	"context"
 	"io"
+	"io/ioutil"
 	"math/rand"
+	"os"
 	"path/filepath"
 	"sort"
 	"testing"
@@ -195,17 +197,47 @@ func TestModifiedIndex(t *testing.T) {
 		Type: restic.IndexFile,
 		Name: "90f838b4ac28735fda8644fe6a08dbc742e57aaf81b30977b4fefa357010eafd",
 	}
-	err := repo.Backend().Load(context.TODO(), h, 0, 0, func(rd io.Reader) error {
-		// save the index again with a modified name so that the hash doesn't match
-		// the content any more
-		h2 := restic.Handle{
-			Type: restic.IndexFile,
-			Name: "80f838b4ac28735fda8644fe6a08dbc742e57aaf81b30977b4fefa357010eafd",
+
+	tmpfile, err := ioutil.TempFile("", "restic-test-mod-index-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer func() {
+		err := tmpfile.Close()
+		if err != nil {
+			t.Fatal(err)
 		}
-		return repo.Backend().Save(context.TODO(), h2, rd)
+
+		err = os.Remove(tmpfile.Name())
+		if err != nil {
+			t.Fatal(err)
+		}
+	}()
+
+	// read the file from the backend
+	err = repo.Backend().Load(context.TODO(), h, 0, 0, func(rd io.Reader) error {
+		_, err := io.Copy(tmpfile, rd)
+		return err
 	})
 	test.OK(t, err)
 
+	// save the index again with a modified name so that the hash doesn't match
+	// the content any more
+	h2 := restic.Handle{
+		Type: restic.IndexFile,
+		Name: "80f838b4ac28735fda8644fe6a08dbc742e57aaf81b30977b4fefa357010eafd",
+	}
+
+	rd, err := restic.NewFileReader(tmpfile)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = repo.Backend().Save(context.TODO(), h2, rd)
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	chkr := checker.New(repo)
 	hints, errs := chkr.LoadIndex(context.TODO())
 	if len(errs) == 0 {