Enables the underlying TCP Listener to be configured and run by the c…

[robeving]

This PR enables the caller to have greater control of the underlying TCPListener implementation by specifying an interface to use. It also makes PausableFtpService public. PausableFtpService is used by a number of classes already and it does a lot of the heavy lifting so callers should make use of it.

In my case I have my own server implementation that I want to use which provides diagnostics, management etc. I can now use a very small class like this to pass through individual TCP connections.

    internal class InternalListener : PausableFtpService, IListenerService
    {
        private readonly Channel<TcpClient> channels = System.Threading.Channels.Channel.CreateBounded<TcpClient>(1);
        private CancellationTokenSource cts;
        private TcpClient client;

        public InternalListener(CancellationTokenSource cts, TcpClient client) : base(cts.Token)
        {
            this.cts = cts;
            this.client = client;
        }

        public ChannelReader<TcpClient> Channel => channels;
        public CancellationToken Token => cts.Token;
        public bool IsCancellationRequested => cts.IsCancellationRequested;
        public event EventHandler<ListenerStartedEventArgs> ListenerStarted;

        public void Cancel(bool throwOnFirstException)
        {
            cts.Cancel(throwOnFirstException);
        }

        public void Dispose()
        {
            cts.Cancel();
            cts.Dispose();
        }

        protected override async Task ExecuteAsync(CancellationToken cancellationToken)
        {
            try
            {
                ListenerStarted?.Invoke(this, new ListenerStartedEventArgs((client.Client.LocalEndPoint as IPEndPoint).Port));
                await channels.Writer.WriteAsync(client, cancellationToken).ConfigureAwait(false);
            }
            catch (Exception ex) when (ex is OperationCanceledException)
            {
                // Ignore - everything is fine
            }
        }
    }

Fixes https://github.com/FubarDevelopment/FtpServer/issues/114

[fubar-coder]

This looks good. I'll take a look why some of the checks failed.