Aggregations: Properly support top_hits aggregation in a nested and reverse_nested aggregations.

At the moment the `top_hits` aggregation fails if it is being put in a nested or reverse_nested aggregations. This PR add proper support for nested inner objects being emitted as hits in the `top_hits` aggregator. All the known fetch phase features are supported.

This PR adds fetch infrastructure to get #3022 in as well.
I tried that, it didn't make things simpler. I still need to go check if the mapper is a nested object mapper and the type filter falls in the scope the nested docId.

I removed the todo for now, I think it is ok.