[IMP] stock_account: change the standard price calculation for FIFO products

Before
======
If a product is FIFO, the standard price is the unit cost of the last released.
```
+ 1 product @ 10€
+ 1 product @ 15€
- 1 product
-> Actual standard price is 10€

```

After
=====
If a product is FIFO, the standard price of the ``product.product`` is the
unit cost of the next product which will  be released
```
+ 1 product @ 10€
+ 1 product @ 15€
- 1 product
-> Actual standard price is 15€

```

Task #2038418