CNDB-16243: Improve ChunkCache Key in order to keep O(1) access time

[eolivelli]

What is the issue

ConcurrentHashMap backing ChunkCache sometimes falls into Treeification ( O(log n) access time instead of O(1))

Full context in https://github.com/riptano/cndb/issues/16243

What does this PR fix and why was it fixed

• Improve hashCode of the Key to prevent collisions
• Implement Comparable in case the ConcurrentHashMap falls back in to Tree mode

[github-actions]

Checklist before you submit for review

• This PR adheres to the Definition of Done
• Make sure there is a PR in the CNDB project updating the Converged Cassandra version
• Use NoSpamLogger for log lines that may appear frequently in the logs
• Verify test results on Butler
• Test coverage for new/modified code is > 80%
• Proper code formatting
• Proper title for each commit staring with the project-issue number, like CNDB-1234
• Each commit has a meaningful description
• Each commit is not very long and contains related changes
• Renames, moves and reformatting are in distinct commits
• All new files should contain the DataStax copyright header instead of the Apache License one

[pkolaczk]

The change looks good. But this is something that can be easily (and cheaply) tested.
Especially it would be nice to reproduce the original issue in the test (like - which keys caused so many hashCollisions), and then demonstrate the the new hash is indeed better.

[eolivelli]

But this is something that can be easily (and cheaply) tested.
I don't know if this is worth to do in a unit test.

I am using this patch in the 1B vectors POC and the ConcurrentHashMap now works much better.

I added unit tests (mostly generated by AI, with some modifications) to validate the implementation.
There is also a small test to try to test the distribution of hashCode

[pkolaczk]

Looks good now

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[cassci-bot]

❌ Build ds-cassandra-pr-gate/PR-2156 rejected by Butler

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1 regressions found
See build details here

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Found 1 new test failures

Test	Explanation	Runs	Upstream
o.a.c.index.sai.cql.VectorCompaction100dTest.testOneToManyCompactionTooManyHoles[db true]	NEW	🔴⚪	0 / 19

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Found 1 known test failures