TDigest: use rocksdb iterator for centroids iteration.

[LindaSummer]

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• I had searched in the issues and found no similar issues.

Motivation

Related Issue

#2558

In TDigest implementation #2741 , we use DummyCentroids to achieve a simple iteration and it should be refactored to rocksdb native iterator as designed.

#2741 (comment)

Solution

• Implement a RocksdbCentroids and replace the DummyCentroids.
• Add more unit tests for encoding, decoding and algorithm.

Are you willing to submit a PR?

• I'm willing to submit a PR!

[LindaSummer]

Hi Team,

I'm working on this issue now. 😊

I will create a new centroid iterator with rocksdb iterator directly.

I will add more unit tests and create some benchmark for quantile.

Best Regards,
Edward