feature request: consolidate batch request

[koshdim]

as shown in example

batch.post(() => {
      airports.get().subscribe(console.log);
      airport.get().subscribe(console.log);
      people.get({withCount: true}).subscribe(console.log);
    }).subscribe();

batch request subscriptions and inner request subscriptions are different, therefore if one needs to wait until all inner requests complete it is a bit difficult to implement, as batch completes faster than inner requests. would be great to have one observable that completes when all inner requests finish