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.
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Status Job ID Name Coverage
  Test
passed #761
testing

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