Reconsider the best way to encode postings now that we can skip non-competitive hits [LUCENE-8760]

The fact that we now skip non competitive hits has some implications to our postings:
 - we are now more likely to call advance vs. nextDoc
 - we are less likely to read term frequency for a given doc, since we only do that if the maximum score reported by impacts is competitive
- we are less likely to read positions for a given doc, since exact phrase queries first check the maximum score that would be obtained with a phrase freq equal to the minimum of all term freqs

It might be a good opportunity to re-explore the best way to encode postings.

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Migrated from [LUCENE-8760](https://issues.apache.org/jira/browse/LUCENE-8760) by Adrien Grand (@jpountz)

No files or tests are named. Start by reviewing Lucene's postings encodings and the advance/nextDoc, term-frequency, positions, and impacts paths described in the issue. Done would require a decided encoding approach and evidence that it suits the changed access patterns.

Written by the indexing model from the issue text.