[FIX] account: multi-currencies fixes

- Create 2 rates for a secondary currency
  2019-01-01: 1.0
  2019-10-01: 2.0
- Create an invoice on 2019-09-01 in the secondary currency
- Register a payment on 2019-10-02: pay the invoice partially in the
  secondary currency, and mark the invoice paid

No journal entry is created for the currency rate difference.

Backport of commits e1306066, 8bec4fa0 and
f7971a95.

opw-2072519

closes odoo/odoo#38252

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>