Decrease lock contention on new nodes in cnc_port_add (inspired by #6).
This function previously would only lock the csw and cport associated with the new node, but it didn't actually lock the node -- it left that to others. I see no reason to allow that to continue; we're going to lock it shortly anyway. And this will prevent contention in the new node path vs ARPs for or from this node in the packet_in path; that is what I thought the original problem in #6 was --- no longer sure if that really was it. Still, this is an improvement, I believe.