UNICAST3: disable retransmission

When run over TCP, we don't really need retransmission enabled in UNICAST3.

However, we still need ordering, as the thread pool can destroy the ordering provided by TCP.

Setting xmit_interval to a value <= 0 disables retransmission. However, we need to make sure that the sender still purges messages seen by the receiver, therefore ACKs still need to be sent from receiver to sender. As this is done in the retransmission task, we now need to send it on reception of a message (perhaps define how many messages/bytes are received before an ACK is sent?)...

Note: when retransmission is disabled, we need to make sure that the thread pool doesn't discard messages when full, ie. that the RejectionPolicy is CallerRuns. This should be done in init(). If the thread pool discarded messages, we would not be able to deliver messages as there are gaps in the retransmission table, but no retransmission is happening.