Copied from Mingle

This is a cleanup task which, if unattended, could bite us if we have to do things with transactions in Civi. It's different than other gateways and makes querying more fiddly. If things break it'll be harder to fix.

The solution is not totally clear-cut, so please consider carefully. Our gateway_txn_id field is being populated with the synthetic orderid-effortid combination, like 1099860561-3. That makes it impossible to search for a recurring transaction by order id, which is one of the main motivations for having this field.