Auto-cancel the moment a doc changes

[jhs]

If you start a long-running transaction and follow up with a short-running one, the shorter will finish first. When the long-runner finally finishes, it will get an inevitable 409 and begin the (slow) process again.

It would be nice to at least immediately detect when a transaction is doomed, and immediately jump to abort or retry code.

Maybe Txn can follow a _changes feed filtered on the _id of the document in question. And if it sees an update, it can immediately abort/retry.