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/backend/sftp/sftp.go

@@ -282,7 +282,7 @@ func Join(parts ...string) string {
 }
 
 // Save stores data in the backend at the handle.
-func (r *SFTP) Save(ctx context.Context, h restic.Handle, rd io.Reader) (err error) {
+func (r *SFTP) Save(ctx context.Context, h restic.Handle, rd restic.RewindReader) error {
 	debug.Log("Save %v", h)
 	if err := r.clientError(); err != nil {
 		return err