Refactor ListenRELP using Netty

The ListenRELP processor currently depends on SocketChannelDispatcher, which uses the custom SSLSocketChannel when enabled for TLS. The addition of Netty-based alternatives in nifi-event-transport removes the need for custom SSLEngine interaction and also streamlines the implementation of custom socket protocols. Refactoring ListenRELP to use Netty should also include updating associated unit tests to improve reliability.