[Sprint 40] XD-2509 Fix classpath issues for RabbitMQ MessageBus (subtask of XD-2503)

- override the ThreadContextClassLoader when creating the RabbitMQ consumer, setting it to the MessageBus classloader, thus allowing the proxies based on SimpleMessageListenerContainer$ContainerDelegate to see the proper class
Don't we want to use `this.getClass().getClassLoader()` or `RabbitMessageBus.class.getClassLoader()` instead?
Also, how can I reproduce a problem before this fix, to test it?