Assert ServerSocketChannel is not Blocking (#43479)

* Assert ServerSocketChannel is not Blocking

* Relates #43387 which appears to run into blocking accept calls

Author: original-brownbear

libs/nio/src/main/java/org/elasticsearch/nio/ChannelFactory.java

@@ -177,7 +177,9 @@ SocketChannel openNioChannel(InetSocketAddress remoteAddress) throws IOException
 
         SocketChannel acceptNioChannel(ServerChannelContext serverContext) throws IOException {
             ServerSocketChannel rawChannel = serverContext.getChannel().getRawChannel();
+            assert rawChannel.isBlocking() == false;
             SocketChannel socketChannel = accept(rawChannel);
+            assert rawChannel.isBlocking() == false;
             if (socketChannel == null) {
                 return null;
             }