message: Don't hold all PutObjectOutputs in memory
diff: @@ -77,7 +77,7 @@ impl<'a> S3Backend<'a> {
  }
  attempts += 1;
  - match self.runtime.block_on(futures.collect()) {
  + match self.runtime.block_on(futures.map(drop).collect()) {
  // this batch was successful, start another batch if there are still more files
  Ok(_) => break,
  Err(err) => {
  