Add flush($timeout) method optional call for sending messages through Enqueue\RdKafka to make it sync

[Vorotyagin-Anton]

In Enqueue\RdKafka\RdKafkaProducer $timeout === 0 is hardcoded in method send() call. So, the only way to wait for delivery report to handle errors is calling flush(-1) method from Enqueue\RdKafka\RdKafkaProducer. But it is not implemented in Enqueue\MessengerAdapter\QueueInteropTransport send method.