DocValueFieldsFetchSubPhase should pull one iterator for all top hits

[jpountz]

Doc values have moved to an iterator API in Lucene 7. We should fix DocValueFieldsFetchSubPhase to only pull the iterator once and then iterate over top hits in doc id order in order to limit the cost of calling advance on the doc value iterators.

[jpountz]

This probably also means we should try to benchmark doc_values fields as we might have to update our recommendations as to when it is a good idea, eg. compared to using _source.