Improve copy local to S3 process: Limit number of files to upload per process.

Moving to won’t fix for s3fs as the 4.x branch has removed the code for file migration in favor of using an “independent” module https://www.drupal.org/project/bfm → to handle file import.

The change allows sites to remove the code if they do not require it and will allow developers working on 4.x to focus on s3fs related tasks while receiving assistance from the rest of the Drupal community for file migration features.