(7.0.z) UNICAST3: bypass or remove when running over TCP

When running over TCP as transport, UNICAST3 is still required: while TCP/IP retransmits messages reliably and also provides sender-FIFO ordering, the receiver's thread pool might be exhausted and thus the message might get rejected.

However, if the regular and OOB thread pools are disabled, we could actually bypass (or completely remove) UNICAST3. If messages get dropped by a protocol further up the stack, however, there will be no retransmission in this case.

SOLUTION:

• Document this behavior
• Emit an INFO message (or automatically bypass UNICAST3) when run over a TCP transport and both OOB and regular pools are disabled