637: Split the node network layer out of the node kernel r=coot a=dcoutts
This should be a pure refactoring, with no changes. It takes the network interface initialisation out of the NodeKernel and moves it into its own layer which can be constructed from the NodeKernel.
This is another step in the direction of fully integrating the remaining network layer features (mux, version negotiation etc).
Co-authored-by: Duncan Coutts [email protected]