[4.x] [RFC] Drop `ResultPagerIterface::postFetch`

[GrahamCampbell]

I think we should remove postFetch from the result pager interface and make the function private on the result pager. What use case is there for calling it from the outside?

[GrahamCampbell]

We could also make classes like ResultPager and ResponseMediator final.

[acrobat]

I will make a PR to mark those classes as final, that's indeed a good idea! I also don't see a direct usecase for the postFetch method, so maybe deprecate it in 3.1 and have it removed in. 4.0. This will give users time to report valid usecases for this method, also keeping the method in for now doesn't really do any harm or increased maintenance.