Discount calculated based on old state of the cart

When a cart that has an applied discount is updated, e.g. an item is added, the discounts are not correctly calculated.

Example:

We have a cart with 1 item that costs 99$ and a 10% discount applied => the discount is 9.90$.
We change the quantity of the same item to 2
After the item is added, the item total becomes 198$ but the discount total still remains 9.90$.

The cart undergoes all the transformations before being saved and one of the transformations is the setDiscountsOnCart. The discounts plugin calls all registered discount codes functions for every discount on the cart: https://github.com/reactioncommerce/api-plugin-discounts/blob/9d1bc40f86cea520f234b3c812c117400c795998/src/queries/getDiscountsTotalForCart.js#L26

The problem is that at this stage the new cart is not yet persisted as not all transformations had finished so in the discount code function we are fetching the last persisted cart from the database, rather than using the cart that is currently being transformed and saved: https://github.com/reactioncommerce/api-plugin-discounts-codes/blob/643c7f7e6594f9539037804a8055e0a531540a84/src/util/getPercentageOffDiscount.js#L23

A solution is to use the cart that is passed to the discount transformation function rather than fetching the cart from the database.

Breaking change

This change would require a change in the discount functions parameters both in the api-plugin-discounts and in api-plugin-discounts-codes