Improvements to migrator

After looking through the migrator to prevent php timeouts during the schema update, I found some other issues that might be undetected yet.

• $output_handler is public, even though there is no need for that
• At the end of a data update, it might have been the case that there was a "Update data completed" although this wasn't the case yet
• Reverting data could have resulted in a loop under some circumstances
• Reverting "if" statements was possible and I think this might have unexpected results so instead this won't be tried anymore with my patch.