Having a big number of assignments the "\assignfeedback_editpdf\task\convert_submissions" task starts lagging.
It also has no limit of records to process so it tries to process the qhole queue in one go, works for enormous amount of time and eventually dies.
As we can't run more than one instance of the scheduled task at the same time and there is no way to speed it up, it seems to be a good idea to convert submissions using adhoc tasks.
As part of this enhancement all existing and not processed records in `mdl_assignfeedback_editpdf_queue` will need to be converted into adhoc tasks. Then the table can be deleted.