LUCENE-9877: Allow up to 7 exceptions in PForUtil (instead of 3)

[gsmiller]

Description

This change allows PForUtil to encode up to 7 exceptions per encoded block (instead of the 3 allowed today) in order to reduce index size.

Solution

I tested the impact of 7 exceptions vs. 3 using a new tool I wrote to directly inspect the contents of PFOR encoded index blocks (details in the Jira) as well as inspected overall index size reduction. I also tested QPS impact using luceneutil benchmark tools. All details are in the Jira issue.

Note that this change should be fully backwards compatible, even though I don't think that needs to be a requirement. Details of this are also mentioned in the Jira issue.

Tests

All existing tests pass. No new tests seemed necessary as TestPForUtil already exercises the code.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Lucene maintainers access to contribute to my PR branch. (optional but recommended)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.

[jpountz]

The change looks good to me. Can you add a CHANGES entry? Also, should we update the 8.x postings format too?

[gsmiller]

Thanks @jpountz. I updated CHANGES.txt just now. As for updating 8.x, I'm a bit naive on how to go about suggesting that change. The change is backwards-compatible so this should be possible. I think this would involve opening an PR against the old repo and then also updating the backwards_codecs version of PForUtil in this PR?

[gsmiller]

I've opened a separate PR to backport this into branch_8x as well since it's fully backwards compatible. I updated this PR to also include the change in "backwards_codecs", assuming that we want to backport. If we decide not to, I can remove that change. Thanks!