Cache-aware choice of `TopNParams.numValuesPerPass`

Currently, the available processing buffer is always used in full:

https://github.com/apache/incubator-druid/blob/566dc8c719489283f9190cefd6346bbb3f12955f/processing/src/main/java/org/apache/druid/query/topn/PooledTopNAlgorithm.java#L255

But this may not be optimal in certain cases, as demonstrated in [this post](https://lemire.me/blog/2019/08/16/faster-threshold-queries-with-cache-sensitive-scancount/) by @lemire.

Start with processing/src/main/java/org/apache/druid/query/topn/PooledTopNAlgorithm.java at the linked buffer-allocation line, then read the linked cache-sensitive threshold-query post. Determine how TopNParams.numValuesPerPass should account for cache behavior instead of always using the full processing buffer; done should include evidence that the revised choice improves the affected query cases.

Written by the indexing model from the issue text.