Revert undesirable lateral

[benwilson512]

#144 introduced an effort to ensure that when a has_many through association was queried from dataloader, the order specified in dataloader is preserved.

Unfortunately per comments on that PR, this introduced some of its own bugs, and wrapping any order by in an a lateral join had problematic performance regressions. #165 also seems related to this.

This PR reverts that change, and alters the relevant test case in Dataloader to use a many_to_many which does show the order preserving properties.

[Wigny]

Hey! We are facing a similar issue to #78 but when using our own custom Dataloader.KV source, in which it fetches all intermediary entities first and later fetches all related entities for each intermediary one. Our issue lies on our final entities are ordered as we want but when resolving the intermediary ones the ordering get lost.

I wonder if you are aware and could point which logic does Ecto perform when using many_to_many with join_through but not when using has_many with through, so we can replicate it when resolving our structs orderly...