Cart session.

Cart session id can be transferred via non-secure connection. So the cart session id must be changed when a customer try to make an order.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
session_id	TEXT		Cart session id. Random string.
cart_id	INTEGER	NOT NULL	Cart ID. (cart.id)

CREATE TABLE cart_session(
shop           TEXT,
session_id     TEXT,
cart_id        INTEGER  NOT NULL,
PRIMARY KEY(shop, session_id)
);