Have PaymentController.redeem redeem each counter value in turn

See https://github.com/PrivateStorageio/PaymentServer/issues/51 for general context

After the counter value is accepted by all necessary Python APIs, persisted to the database, and visible on the voucher state representation, the controller should iterate over valid counter values and perform redemption with each in turn.

There is no major benefit to trying to do any of this work concurrently since the very first group of ZKAPs will let the client start doing work and redemption is resource intensive (so, eg, we would want to be careful about how concurrently we try to do it to avoid negatively impacting interactive performance on the client machine).