Accept a counter value at all API points in the redemption process

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

As a first step in having the client perform a multi-step redemption using counters, make it possible to pass a counter value to all of the necessary parts of the implementation. The counter value should eventually be sent over the network with the redemption request. The value should always be 0 for this ticket. Actual counting up will come in a follow-up.