Refactor out the model into db

api/api.go

api/auth.go

api/auth_test.go

api/db/db.go

api/db/errors.go

api/db/member.go

api/db/order.go

api/db/product.go

api/db/transaction.go

api/mail.go

api/member.go

api/member_test.go

api/order.go

api/order_test.go

api/product.go

api/product_test.go

api/purchase.go

api/purchase_test.go

api/topup.go

api/topup_test.go