Message queue for the sync

[felipeelia]

Description of the Change

This PR fixes the problem outlined in #3510 by adding a small message queue to the sync script.

A few things to note:

1. The sync script accepts a total_attempts parameter. The script will try that number of times to sync a batch;
2. For the dashboard sync, total_attemps it 3;
3. As the dashboard sync relies on several AJAX calls, the process is interrupted and started every time it calls the output method (so the AJAX request is replied to with a message.)
4. When we output during one of those tries, the script exits earlier than we need, not setting the last processed item ID properly

With the message queue in place, we avoid calling the output method in the middle of the process.

Closes #3510

Changelog Entry

Fixed - Same error message being displayed more than once on the Dashboard sync

Credits

Props @felipeelia, @MARQAS, @tott, and @wildberrylillet.

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.