[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
You should use a read_group on lines to have the count for each partner.