Adapt high-level components to new parent API

This patch adjusts the various users of the 'Child' API to the changes
on the account of the new non-blocking parent interface. It also removes
the use of the no-longer-available 'Connection::KEEP_OPEN' feature.

With the adjustment, we took the opportunity to redesign several
components to fit the non-blocking execution model much better, in
particular the demo applications.

Issue #2120

repos/demo/include/scout/event.h

@@ -76,7 +76,7 @@ class Scout::Event_handler
 		/**
 		 * Handle event
 		 */
-		virtual void handle(Event &e) = 0;
+		virtual void handle_event(Event const &e) = 0;
 };
 
 #endif /* _INCLUDE__SCOUT__EVENT_H_ */