Fix flaw in presenter that could cause unintended shared state.

Implementing this will break the `mvp-sample` here: https://github.com/peholmst/vaadin4spring/tree/master/samples/mvp-sample.  However, the fix is required so that we don't a) limit developer from using other `EventScope` with `EventBus#publish`, and b) the way the sample is structured, state is shared across browser sessions, not entirely desirable.  

I will prop updates to the `mvp-sample` to make it functional again once you accept this request.