[IMP] membership: work on duration, not on date

Task: https://www.odoo.com/web#id=6806&view_type=form&model=project.task&action=333&active_id=131
Pad: https://pad.odoo.com/p/r.yIMRe1VVusFuPpnV
use `self.pool['membership.membership_line']` (we DO want to crash if the pool is buggy)