FieldComparators Should Not Maintain Implicit PQs [LUCENE-8950]

While doing some perf tests, I realised that FieldComparators inherently maintain implicit priority queues for maintaining the sorted order of documents for the given sort order. This is wasteful especially in the case of a multi feature sort order and a large number of hits requested.

We should change this to have FieldComparators maintain only the top and bottom values, and use them as barriers to compare

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Migrated from [LUCENE-8950](https://issues.apache.org/jira/browse/LUCENE-8950) by Atri Sharma (@atris)

Start by locating the FieldComparators implementation and reading how it maintains implicit priority queues for sorted hits. Review the existing performance tests mentioned in the issue, if present. Done means FieldComparators maintain only top and bottom values and use them as comparison barriers without changing the required sort behavior.

Written by the indexing model from the issue text.