Partially executed multi-result queries are not treated as partial results

Given the following query:

BEGIN NOT ATOMIC

  SELECT SLEEP(1);

  SELECT SLEEP(2);

  SELECT SLEEP(3);

END;

If the server on which the query is being executed is killed after the first SELECT has returned its result but the second query has not returned any parts of its result, the fact that a statement was partially executed is not detected.

In debug builds this manifests as a debug assertion inside RWSplitSession::retry_master_query but in release builds this causes an extra error to be written to the client that it does not expect.