Failure when closing challenge

@afrisalyp - I think we get a lot of failures when closing challenges with errors like this:

Payment failed: [400]: Failed to update challenge. Detail: javax.ejb.EJBTransactionRolledbackException

We've experienced that for awhile, and from what I can figure out, I think it happens when we end up trying to process the same ticket twice, at the same time. Can you double check if that could happen please? Maybe we can add a lock on the record in DynamoDB to indicate a record is being paid so we don't attempt to duplicate the payment?

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Git issue link: https://github.com/topcoder-platform/topcoder-x-ui/issues/244