Refund a payment

Moves a posted payment into a Product's pool back to the account that paid it, in full or in part, as a correction of the payment. A full refund first gives back the earn-rate fees taken on the payment; a partial refund leaves them in place. A payment takes one refund: a second refund, or a refund of a payment that already has a reversal or return, is refused with transfer_lineage_conflict naming the existing transfer, and a payment with a return in progress is refused with transfer_return_exists. Platform fee transfers and payments into any account other than a Product pool cannot be refunded here.