KAFKA-6303: Potential lack of synchronization in NioEchoServer

[shivsantham]

In the run() method:

                            SocketChannel socketChannel = ((ServerSocketChannel) key.channel()).accept();
                            socketChannel.configureBlocking(false);
                            newChannels.add(socketChannel);

Modification to newChannels should be protected by synchronized block.

• [ ] Verify design and implementation
• [x] Verify test coverage and CI build status
• [ ] Verify documentation (including upgrade notes)

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