"Transaction closed" timestamp?

[sde1000]

At the moment, transactions have a simple open/closed flag. If we want to know when the transaction was closed we can try to infer it from the date of the most recent transaction line or payment, but we can't be sure. (This may be incorrect in the case of transactions being merged, for example: the merge may result in a closed transaction from multiple open ones, without adding any further translines or payments.)

Some situations have arisen where this would be useful for debugging (eg. "when did this transaction close, and why wasn't it immediately after a payment") and there may be more situations where it's useful when trying to work out what happened.

Would adding a timestamp be sensible? Or would we be better off working on a more general transaction logging system for tracking merges, etc. as discussed in #215?