io_context 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);
function blocks until all work has finished and there are no more handlers
to be dispatched, until the
io_context 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.