Minimize the number of ACK sent. By default, only the highest message of a message batch should be acked, e.g. for 1 4 2 5 3 6 8, 6 should get ack'ed, which would ack all seqnos smaller than 6 as well. We should then not get or send acks for 1-5 (assuming we received 1-8 as a batch)