Automated backups do not resume with unfinished courses

Problem

Since version 3.3 we observe that backup of unfinished courses are not resumed in the next run.

Reproduce:

1. create a big course
2. execute php56 admin/cli/automated_backups.php
3. cancel the job (using ctrl-c) while backing up the big course
4. investigate backup log that the course in question is "unfinished"
5. execute php56 admin/cli/automated_backups.php again, it should resume the backup, but it does not.

Note that documentation and FAQ is not in sync:

https://docs.moodle.org/33/en/Backup_and_restore_FAQ says that unfinished backups are skipped next time 

while https://docs.moodle.org/33/en/Automated_course_backup says that unfinished backups are completed next time

Request

Admin sections should have a script to reconcile automatic backups: (cleanup temp; clean entries in database mdl_backup_courses, resp.