Allow to cancel connect() operations when using io_uring (#237)

Motivation:

Whe using io_uring we can in fact cancel a connect() operation by just
closing the underyling fd.

Modifications:

Don't mark the promise as uncancellable before issue the non-blocking
connect() call Add comment to explain code

Result:

Related to netty/netty#13843 and
netty/netty#13849

transport-classes-io_uring/src/main/java/io/netty/incubator/channel/uring/AbstractIOUringChannel.java

@@ -680,7 +680,9 @@ void connectComplete(int res) {
         @Override
         public void connect(
                 final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
-            if (!promise.setUncancellable() || !ensureOpen(promise)) {
+            // Don't mark the connect promise as uncancellable as in fact we can cancel it as it is using
+            // non-blocking io.
+            if (promise.isDone() || !ensureOpen(promise)) {
                 return;
             }
 
@@ -760,6 +762,8 @@ public void run() {
             promise.addListener(new ChannelFutureListener() {
                 @Override
                 public void operationComplete(ChannelFuture future) {
+                    // If the connect future is cancelled we also cancel the timeout and close the
+                    // underlying socket.
                     if (future.isCancelled()) {
                         cancelConnectTimeoutFuture();
                         connectPromise = null;