add a StreamContext to the Pre/PostSource signals

 - fill the StreamContext in the Eventprocessor before reading the Event
 - pass the StreamContext to the Pre/PostSource signals
 - drop the EventSourceProxy, and call the Pre/PostSource signals
   explicitly from the InputSource
 - adapt all services to the change in the signals
reintroduce and use the EventSourceSentry
add a StreamContext to the Pre/PostSource signals
re-enable the pre/post/source measurements