Use payment transaction payload for additional diagnostics/info.

A commerce payment transaction entity has a 'payload' property, which can hold information returned by a payment provider, useful when debugging or just to provide additional information about a transaction to site administrators (for example, when using Ogone, which payment method offered by Ogone was used?). Commerce already displays this information to users with the "Administer Payments"-permission:

The patch attached saves Ogone's feedback in the transaction's payload property. Feedback handling was slightly altered to remove $_SESSION parameters and $_GET['q'] to prevent those from getting included in the payload.