Backport MDL-57610 - scheduled task 'convert_submissions' should have a date/attempt check

Thanks to the great work from Adam Olley & peer review by Damyon, the Moodle conversion task queue can now handle failures much better.

I would strongly suggest that this gets backported to 3.4 & 3.5, please. Conversions are causing major operational issues at scale. Investigating things today we had 18 000+ unprocessed rows in the queue as a result of the lack of attempt checks (task initially got stuck a month ago). We are currently on 3.4 and planning to move to 3.5 LTS in summer 2019 with a view to remain on it until it's EOL in 2021. I don't think our users can accept that these conversion failures for the Moodle assignment remain uncaught for another 3 years if this major fix doesn't get backported.

I see Damyon mentionned there was a new admin setting which was preventing the backport. If that's the case, the backport could simply ditch the setting and just stick to the default 3 failures required to get removed from the queue.

Much appreciated.

Alistair