txn: ignore changes after rollback on yield

Previously, if a yield occurs for a transaction that does not
support it, we roll back all its statements, but still process
its new statements (they will roll back with each yield). Also,
the transaction will be rolled back when a commit is attempted.
Now we stop processing any new statements right after first yield,
if transaction doesn't support it. This is done so that when we
implement timeout for transactions, the rollback of the transaction
at its expiration and at yield has the same behavior.

Part of #6177