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/cache/backend.go

@@ -5,7 +5,6 @@ import (
 	"io"
 	"sync"
 
-	"github.com/pkg/errors"
 	"github.com/restic/restic/internal/debug"
 	"github.com/restic/restic/internal/restic"
 )
@@ -50,35 +49,29 @@ var autoCacheTypes = map[restic.FileType]struct{}{
 }
 
 // Save stores a new file in the backend and the cache.
-func (b *Backend) Save(ctx context.Context, h restic.Handle, rd io.Reader) (err error) {
+func (b *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader) error {
 	if _, ok := autoCacheTypes[h.Type]; !ok {
 		return b.Backend.Save(ctx, h, rd)
 	}
 
 	debug.Log("Save(%v): auto-store in the cache", h)
 
-	seeker, ok := rd.(io.Seeker)
-	if !ok {
-		return errors.New("reader is not a seeker")
-	}
-
-	pos, err := seeker.Seek(0, io.SeekCurrent)
+	// make sure the reader is at the start
+	err := rd.Rewind()
 	if err != nil {
-		return errors.Wrapf(err, "Seek")
-	}
-
-	if pos != 0 {
-		return errors.Errorf("reader is not rewind (pos %d)", pos)
+		return err
 	}
 
+	// first, save in the backend
 	err = b.Backend.Save(ctx, h, rd)
 	if err != nil {
 		return err
 	}
 
-	_, err = seeker.Seek(pos, io.SeekStart)
+	// next, save in the cache
+	err = rd.Rewind()
 	if err != nil {
-		return errors.Wrapf(err, "Seek")
+		return err
 	}
 
 	err = b.Cache.Save(h, rd)