Simplify TransportBroadcastByNodeAction

Similarly to elastic#92987, recent improvements to the primitives for writing
async code (particularly elastic#92452 and elastic#92620) mean that we can enormously
simplify `TransportBroadcastByNodeAction`. In particular, we can avoid
accumulating an intermediate array of responses for later processing in
favour of just accumulating the successes and failures into their final
separate lists. We also no longer need to use a separate
`NodesResponseTracker` to discard responses on cancellation. Finally, we
can now discard shard-level responses more promptly on cancellation.

Author: DaveCTurner