Add counter to the voucher table

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

After #121 (closed) there will be a counter value supplied to each part of the redemption process. To make it possible to reliably count this counter up, it will need to be persisted in the database. Add a field to the voucher table for keeping track of the value on a per-voucher basis. This will make it possible to reliably resume redemption even if the process is restarted partway through.