fix instant RST/FIN after SYN/SYN-ACK/ACK

[adiantek]

Motivation:

If you close the connection before the backend connects, the backend will keep connected until timeout. If you add logging, you will have sth like that:

12:57:57.398 ProxyFrontendHandler channelActive
12:57:57.405 ProxyFrontendHandler channelInactive
12:57:57.431 ProxyBackendHandler channelActive

Ping between backend and frontend was about 50 ms, and the time difference between connecting and disconnecting was about 1 ms.
Example code in Java to test frontend/backend:

        while (true) {
            System.out.println("tick");
            try {
                Socket socket = new Socket();
                socket.connect(new InetSocketAddress("127.0.0.1", 8443), 3000);
                socket.close();
                Thread.sleep(100);
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

PS. Why ChannelFuture from new Bootstrap().connect is not cancellable? In epoll you can simply close FD.

Modification:

Close backend connection if inbound is not active.

[normanmaurer]

Thanks!