Improve topN algorithm

## Description

@leerho proposed how the topN algorithm can be improved here: https://github.com/apache/incubator-druid/issues/7161#issuecomment-468890906

## Motivation

Avoid potentially significant errors, see #1134 and #1135.

Start with the proposal in issue #7161, especially the referenced comment, then read issues #1134 and #1135 for the errors motivating the change. Establish the intended topN behavior and acceptance criteria from those discussions before determining what implementation and validation are needed; the issue does not name files or tests.

Written by the indexing model from the issue text.