Split from #88 : what is needed is just the TCP network code ported to MS Windows as per ​ossbuild ticket 166

The new connection is like a brand new session (re-use all the attributes), but it needs to hang off the one it is shadowing

At the moment, xpra supports this on the server: the xpra server stays alive even when shifter_server is stopped.
Ideally, we want to keep all the sessions running and just re-load the session files when the server re-starts. The problem comes from the fact that NX writes to stdout and we watch this output to get the current session state, we will need to redirect to a file instead and watch that (and maybe also start it with 'nohup'?)