Basically this fixes MDL-70833 that was reproduced while
testing the update_enrolments requests. Summary:
- Fixes a typo in table name preventing it to be updated ever.
- Fix outer join that was missing records.
- Stop playing and mixing ids (local and remote).
- Better control which enrolments have to be kept (previously
they were being deleted immediately after creating them).
- Improve the coding style of inserts.
- Modernise the deletions to use sql helper and to work with 0..n ids.
478f423e2d