Abandons a date change that is still waiting for its difference
payment, releasing the order so another date can be chosen. The
original tickets are untouched and nothing is charged.

fulfillment_order_id is the id returned by POST /fulfillment/reorder.

Idempotent: cancelling an already-cancelled date change succeeds.
A date change whose difference was already captured cannot be
cancelled (409) — returning that money is a refund, not a cancel.

Starting a new date change on the same order also supersedes an unpaid
one automatically, so calling this first is optional.