* Minimize PostAggregator computations
Since a change back in 2014, the topN query has been computing
all PostAggregators on all intermediate responses from leaf nodes
to brokers. This generates significant slow downs for queries
with relatively expensive PostAggregators. This change rewrites
the query that is pushed down to only have the minimal set of
PostAggregators such that it is impossible for downstream
processing to do too much work. The final PostAggregators are
applied at the very end.
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