object's event processing loop for a specified duration.
template< typename Rep, typename Period> std::size_t run_for( const chrono::duration< Rep, Period > & rel_time);
run_for() function blocks until all work has finished
and there are no more handlers to be dispatched, until the
has been stopped, or until the specified duration has elapsed.
The duration for which the call may block.
The number of handlers that were executed.