Queue cancellation of the trade matched on a request, after a quote has been
accepted. Only the two trade parties can cancel: the requester and the responder
whose quote was accepted. The RFQ engine validates and applies it asynchronously:
it returns any funds already allocated to the trade and voids it, and the request
then reads as cancelled.

A trade can be cancelled only until both parties have allocated: once both sides have
funded, the trade is committed to settle and neither party can back out. A caller who
can see the request but is not a trade party gets 409 trade_not_cancellable, before
and after the match. A trade party whose trade is not created yet gets 404
not_matched; retry shortly.