Update EventHandlingConfiguration to EventProcessingConfiguration

[smcvb]

The EventHandlingConfiguration has been replaced in favor of the EventProcessingConfiguration.
Thus all notion of the EventHandlingConfiguration should be replaced, as well as adding a description of the new features it allows (configuring several Sagas in one Event Processor for example).
See #623 for the implementation.

[abuijze]

We should remove the reference to using multiple instances of EventHandlingConfiguration:

Typically, an application will have a single EventHandlingConfiguration defined, but larger more modular applications may choose to define one per module.

While it's possible to have multiple instances, you may find side-effects as described in AxonFramework/AxonFramework#731.