Add publisher liveliness lost event

[ivanpauno]

After going a bit on circles, I think I have found an approach I'm happy with.

I don't like much the approach of passing a Class<T> and relying on runtime reflection. That's usually a java anti-pattern (except when you really need it).
Instead of passing a class, I'm passing an "event status factory" (that's what actually needed).

I'm also trying to avoid giving the user the possibility of shooting in their own feet using dispose
e.g.:

node.dispose();
publisher.dispose(); // the node was already disposed, the publisher will use an invalid node handle -> UB

That's kind of bad ....
I tried to solve the problem by ensuring that:

• publisher.dispose() will also dispose it's child events.
• doing event_handler.dispose() twice isn't a problem.

I guess that's a better pattern ...

There will still be a problem when disposing while the EventHandler is used by an Executor.
Handling that correctly will require a big refactor, which is not the goal here (I only tried to avoid repeating the pre-existing problems where it was easy to do so).

Next step:

• Adding support to events in Executor.

[ivanpauno]

This is targeting feature/events branch, as the code isn't really usable yet (no executor support)

[jacobperron]

The approach looks good to me. Some minor comments and suggestions below.

[ivanpauno]

Merged on the feature branch, thanks for the review @jacobperron.
Next step: support in the executor.