Major Description
In UNICAST3, if ack_batches_immediately is true, a message batch is acked before delivering it. So this is basically acking on reception rather than delivery.
However, this is not done for single messages. I could change the ack_batches_immediately to ack_on_reception, and - if true - ack single messages also before they're delivered.
Thus we'd have a single property which governs whether single messages or message batches are acked on reception or delivery.
