Stop a booking

What a stop can still do depends on how far the booking has got. status is stopped (it had not started: nothing bought, hold released), stopping (the agent stops at its next step, usually within a minute, and the booking ends failed with the hold released unless it had already pressed pay), too_late (the seller has been paid), with_a_person (a person is finishing it by hand; the stop is recorded for them) or already_ended. Keep polling for the final state.