LPS-31462 Message broadcasting in staging

Hey Mike,

here comes the slightly revisited messaging for staging.

I was thinking about the message batching, but it raises several questions which I can answer but it's not that simple to refactor that framework to support that (without hacking)

So I slightly changed the approach - we will have different levels - process, content group, model levels and there will be a property to set if we want to filter out some of them.

There is a factory framework for the messaging, and also a handler framework. The handler can provide the property checking and sending the message, so it's all wrapped, nice and clean.

Note that the classes are not complete, they are skeleton classes.

I've added a proxy bean on the top of this to be able to support spring advice classes. I've added two advice currently one is for error handling, one is for sending out the messages. I think this is the perfect example for cross cutting concerns to be implemented using AOP. I was thinking about the performance side as well, but this won't hurt I think.

Let me know what do you think.

Thanks,

Máté