AsyncRequestsSender (ARS) can fail to send some requests before returning results, but MultiStatementTransactionRequestsSender (and maybe other clients) assume that ARS will send all requests before returning results.

If we decide that the current ARS behavior is correct, in addition to clarifying the contract in comments, we should create bugs to improve MultiStatementTransactionRequestsSender (and maybe other clients) to handle cases when ARS does not send requests, or add an optional way for ARS to wait to return results until all requests have been sent.