So AcknowledgeRequestMsg is externalized with no version.

Which means I can't just add a boolean flag to the message without potentially
corrupting the stream.
The boolean is to say whether the message requires a reply.

Unless somebody knows a way to do this (or can think of something clever :-),
I'm going to have to send replies to NACKs all the time (to support old clients)
even though new clients just ignore it:

ServerSocketManagerHandler

 case m_acknowledge:
 AcknowledgementRequestMsg ackmsg = (AcknowledgementRequestMsg) msg;
 AcknowledgementRequest ack = ackmsg.getAck();
 server.acknowledge(connectionToken, ack);
 // We send the reply although on newer clients it is ignored.
 socketMgr.sendReply(msg);
 break;

UILClientILService:

 case m_acknowledge:
 // We have to ignore NACK replies because of backwards compatibility
 break;

I would have preferred to do something like:

 if (msg.isOneWay() == false)
 socketMgr.sendReply(msg);

With newer clients setting "OneWay" to true on the original send of the message.