Improve receipts handling

[jimthematrix]

Currently receipts are not produced and saved in the receipts DB until after the transaction has completed. This can be confusing when the client attempts to get the state of the transaction by querying on the receipt ID, where 404 will be returned.

A better design would be for the receipt to be saved in the DB with a pending state in the meanwhile, which gets updated to TransactionSuccess or TransactionFailure upon completion.