Limiting number of files transferred at once

Issue:

```
Aug 24 09:02:39 723ecc46ca7f ecs_site_prod: [WAR] bd (module/aws/sdk/aws-autoloader.php:716) require_once(/var/www/site/module/aws/sdk/GuzzleHttp/Exception/ConnectException.php): failed to open stream: Too many open files
```
Suggestion:
adding setting  for $batchsize just like Moodle's version: https://github.com/catalyst/moodle-tool_objectfs/search?q=batchsize

