Allow checking the type of the BeginEvent when handling a EndEvent

Right now there's no information stored about the BeginEvent corresponding to a EndEvent. As a consequence we remove from the stack of event any BeginEvent we found when we met a EndEvent. So in few cases it might happen that we removed the wrong BeginEnd, if for some reason another BeginEvent has been lost.

To avoid that, it would be useful to be able to identify a matching BeginEvent for a given EndEvent and use that information in the observation context listener.