Instead of playing the game of casting between stream_event and derived
class. We overload handle_stream_event with derived stream_event class.
virtual void handle_stream_event(session_complete_event event) {}
virtual void handle_stream_event(progress_event event) {}
virtual void handle_stream_event(session_prepared_event event) {}
Also, make the virtual function non pure virtual, so user can override
the interested event only without defining all of the three.
