Cart information.

It is better to delete the cart by elapsed time from last accessing time.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
id	INTEGER		Identifier. (1-origin)
modified	TIMESTAMP	NOT NULL	Last accessing time. (FIXME: This name would be not appropriate.)

CREATE TABLE cart(
shop       TEXT,
id         INTEGER,
modified   TIMESTAMP NOT NULL,
PRIMARY KEY(shop, id));