Login session.

For security reason, login session should be initialized to NULL when a customer access via not secure connection.

Column	Type	Constraint	Description
shop	TEXT	PRIMARY KEY	Shop ID. (shop.username)
session_id	TEXT		Login session ID. Random string. This should not be changed while a customer making a order because it can be used to identify a temporal credit card information (see session_credit.
account_id	INTEGER	NOT NULL	Account ID. (account.id)

CREATE TABLE login_session(
shop           TEXT,
session_id     TEXT,
account_id     INTEGER  NOT NULL,
PRIMARY KEY(shop, session_id)
);