Split out 'socket' hosting model into a separate optional NuGet package, since most developers won't need it

src/Microsoft.AspNetCore.NodeServices.Sockets/PhysicalConnections/UnixDomainSocketConnection.cs

@@ -0,0 +1,51 @@
+using System.IO;
+using System.Net.Sockets;
+using System.Threading.Tasks;
+
+namespace Microsoft.AspNetCore.NodeServices.Sockets.PhysicalConnections
+{
+    internal class UnixDomainSocketConnection : StreamConnection
+    {
+        private bool _disposedValue = false;
+        private NetworkStream _networkStream;
+        private Socket _socket;
+
+#if NET451
+        public override Task<Stream> Open(string address)
+        {
+            // The 'null' assignments avoid the compiler warnings about unassigned fields.
+            // Note that this whole class isn't supported on .NET 4.5.1, since that's not cross-platform.
+            _networkStream = null;
+            _socket = null;
+            throw new System.PlatformNotSupportedException();
+        }
+#else
+        public override async Task<Stream> Open(string address)
+        {
+            var endPoint = new UnixDomainSocketEndPoint("/tmp/" + address);
+            _socket = new Socket(endPoint.AddressFamily, SocketType.Stream, ProtocolType.Unspecified);
+            await _socket.ConnectAsync(endPoint).ConfigureAwait(false);
+            _networkStream = new NetworkStream(_socket);
+            return _networkStream;
+        }
+#endif
+
+        public override void Dispose()
+        {
+            if (!_disposedValue)
+            {
+                if (_networkStream != null)
+                {
+                    _networkStream.Dispose();
+                }
+
+                if (_socket != null)
+                {
+                    _socket.Dispose();
+                }
+
+                _disposedValue = true;
+            }
+        }
+    }
+}