When a membership got cancelled and, later, restarted, we still had a cancellation date set, leading to the sensation that it was still cancelled.

With this refactor, when a cancellation date is earlier than a future start date, cancellation info is removed from the partner.

BTW I improved a few things:
- Removed old dead code that is no longer necessary in v16.
- Fix a test that was checking what happened when a membership line was cancelled before its start date. It seems a wrong use case because memberships are only supposed to be cancelled during the membership, and not before starting it.
- Free members need to be considered members too.
- Migration script.

@moduon MT-8237

Before this fix, when creating many memberships at once, only the last one was really created.

@moduon MT-8237