An abandoned gateway session locks the cart forever, with no way to clear it

[ishad05]

Steps to reproduce

1. Start a card checkout and abandon it (close the gateway tab).
2. Return to the store and press PROCEED TO CHECKOUT.

Expected: Either the stale session is expired automatically, or the shopper is offered a cancel action.
Actual: A payment is already in progress for this order. Finish or cancel it before changing your cart. The cart is bricked permanently.