private Result search(Query query, Execution execution, int offset, int hits) {
        query.setOffset(offset);
        query.setHits(hits);
        return execution.search(query);
    }
First fetch result from the next searcher in the chain.
If collapse is active, do collapsing.
Otherwise, act as a simple pass through
container-search/src/main/java/com/yahoo/prelude/searcher/FieldCollapsingSearcher.java