minor bugfix to server spawn code
minor bugfix to server spawn code

The child process of the server spawn code used to exit with 'raise SystemExit(0)'; this is wrong because we don't want the parent process' cleanup code (atexit, context managers, etc) running twice. Conceptually, forking the client process is like exec(), and should never cleanup/return. Therefore we exit now with os._exit()

yaniv-aknin/labour