Improve cancellability in TransportTasksAction

Each `TransportTasksAction` fans-out to multiple nodes, accumulates
responses and retains them until all the nodes have responded, and then
converts the responses into a final result.

Similarly to elastic#92987 and elastic#93484, we should accumulate the responses in a
structure that doesn't require so much copying later on, and should drop
the received responses if the task is cancelled while some nodes'
responses are still pending.

Author: DaveCTurner