Server Shutdown doesn't work if client is connected

[switschel]

Currently it is not implemented to shutdown the TCP socket if a client is still connected.
CTRL + C prints that the Server will be shut down but will wait forever.

https://github.com/SoftwareAG/cumulocity-remote-access-local-proxy/blob/5314b80365c82626a17561acc4f57d209c757b80/c8ylp/tcp_socket/tcp_server.py#L155-L159

[switschel]

https://docs.python.org/3/library/socketserver.html
shutdown()
Tell the serve_forever() loop to stop and wait until it does. shutdown() must be called while serve_forever() is running in a different thread otherwise it will deadlock.

[switschel]

Fixed with #39