When a newer request retires the ones started before it.
'on-start': as it starts, so only one request is ever in flight.
'on-answer': as its answer arrives, so the older ones keep running and each answer that
lands first is still shown. Use it when requests start faster than they answer, such as a
route re-planned during a drag, where 'on-start' would show nothing until the drag pauses.
When a newer request retires the ones started before it.
'on-start': as it starts, so only one request is ever in flight.'on-answer': as its answer arrives, so the older ones keep running and each answer that lands first is still shown. Use it when requests start faster than they answer, such as a route re-planned during a drag, where'on-start'would show nothing until the drag pauses.