lib/rest: fix multipart uploads stopping on context cancel

Before this change when the context was cancelled (due to
--max-duration for example) this could deadlock when uploading
multipart uploads.

This change fixes the problem by introducing another go routine to
monitor the context and close the pipe with an error when the context
errors.

backend/pcloud/pcloud.go

@@ -1137,7 +1137,7 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	// opts.Body=0), so upload it as a multipart form POST with
 	// Content-Length set.
 	if size == 0 {
-		formReader, contentType, overhead, err := rest.MultipartUpload(in, opts.Parameters, "content", leaf)
+		formReader, contentType, overhead, err := rest.MultipartUpload(ctx, in, opts.Parameters, "content", leaf)
 		if err != nil {
 			return errors.Wrap(err, "failed to make multipart upload for 0 length file")
 		}