Fix repeat PxPay transactions

@odorcicd and @edward to review.

When we make a ``GenerateRequest`` call to PxPay and provide a value for ``TxnId`` field, repeat use of that same ``TxnId`` does not change the amount of authorization. This makes it impossible for customers to go back and modify the cart in any way (add coupon, change shipping, modify quantities). This PR moves order identifier into ``MerchantReference`` and removes ``TxnId``, which is not required.

Note, transactions in-flight at the time of deploy may be affected, but the volume isn't huge to warrant a more complicated deploy.