#26 Rethink event handling

[Lars Heuer]

- Event system currently blocks (all handlers must handle the event)
- Events are reported before a modification happens (necessary for identity / Topic Maps constraint checking)

Proposal:
- Events should be handled by an instance that checks Topic Maps constraints (blocking op)
- After constraint checking the event is passed to an instance that delivers the events to other subscribers (non-blocking), i.e. to the indexes

- Maybe a before change / after change notification system, indexes need only the after change notification, Topic Maps constraints must be checked in the before change notification