Fix integration test failures under .net 4.7.2

If a socket is disposed after 'ConnectAsync' is invoked but before the
native 'connect' begins, you can get an 'unexpected'
objectdisposedexceptionw which causes the SocketAsyncEventArgs object to
be left in the 'operation is in progress' state forever.

This doesn't happen under newer frameworks, so use a fallback
implementation for .NET 4.7.2 and netstandard 2.0/2.1.

src/MonoTorrent.Connections/MonoTorrent.Connections/SocketConnector.cs

@@ -71,6 +71,30 @@ static void HandleOperationCompleted (object? sender, SocketAsyncEventArgs e)
         }
 
 
+#if NETSTANDARD2_0 || NETSTANDARD2_1 || NET472
+        public async ReusableTask<Socket> ConnectAsync (Uri uri, CancellationToken token)
+        {
+            var socket = new Socket ((uri.Scheme == "ipv4") ? AddressFamily.InterNetwork : AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
+            var endPoint = new IPEndPoint (IPAddress.Parse (uri.Host), uri.Port);
+
+            using var registration = token.Register (SocketDisposer, socket);
+
+            try {
+                // `Socket.ConnectAsync (SocketAsyncEventArgs)` cannot be safely used under .NET 4.7.2.
+                // .NET 4.7.2 has a bug whereby disposing a socket (so it's safehandle is invalid) before the async operation has fully begun
+                // causes the 'SocketAsyncEventArgs' to be left in an inconsistent state (permanently in the 'operation in progress' state)
+                // so it cannot be reused. Work around it by using the synchronous implementation.
+                //
+                // This issue caused random integration test deadlocks/hangs under .NET 4.7.2 as socket connections couldn't be made.
+                await new ThreadSwitcher ();
+                socket.Connect (endPoint);
+            } catch {
+                socket.Dispose ();
+                throw;
+            }
+            return socket;
+        }
+#else
         public async ReusableTask<Socket> ConnectAsync (Uri uri, CancellationToken token)
         {
             var socket = new Socket ((uri.Scheme == "ipv4") ? AddressFamily.InterNetwork : AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
@@ -98,5 +122,6 @@ public async ReusableTask<Socket> ConnectAsync (Uri uri, CancellationToken token
             }
             return socket;
         }
+#endif
     }
 }