prevent multiple instances of TcpListener causing Socket Exception

[meta-meta]

On Unity version 2020.3.16f1, enter->leave->enter play mode will produce a Socket Exception in the console. It seems there's a zombie instance of TcpListener in a thread still listening on 3722 while Arcadia tries to start up nrepl again. Restarting Unity seems to be the only workaround when this happens.

Maintaining a single instance of listener as in this change is resilient to many play/stop/play cycles. I don't know enough about Arcadia's architecture or threading in .NET to determine if this would cause other problems 😄 but so far it's working for me.