private Query getQueryKeyClone(Query query) {
        Query key = query.clone();
        key.setWindow(0, 0);
        key.getModel().setSources("");
        return key;
    }
Returns a query clone which has sourcr, offset and hits set to null. This is used by access to
the maps using the query as key to achieve lookup independent of offset/hits value
container-search/src/main/java/com/yahoo/search/searchchain/testutil/DocumentSourceSearcher.java