Information of products in the cart.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
cart_id	INTEGER		Cart ID. (cart.id)
index	INTEGER		Index of products in added order. 0-origin.
product_id	INTEGER	NOT NULL	Product ID. (product.id)
quantity	INTEGER	NOT NULL	Quantity.

CREATE TABLE cart_product(
shop       TEXT,
cart_id    INTEGER,
index      INTEGER,
product_id INTEGER NOT NULL,
quantity   INTEGER NOT NULL,
PRIMARY KEY (shop, cart_id, index));