CASSANDRA-21021 max level for ZstdDictionaryCompressor is 9

[smiklosovic]

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The Cassandra Jira

[yifan-c]

I would like to share more context.
Looking at https://github.com/facebook/zstd/blob/dev/lib/compress/clevels.h, the srcSize plays a big role in memory consumption. srcSize is the size of the uncompressed data.
In the case of smaller than 16 KiB, even level 22 does not use much memory.
I am not sure what best decision to make :) Would like to here from you too.

[yifan-c]

Thank you for making ZstdCompressor backward compatible!

Maybe add a comment to explain why there are 2 different BEST_COMPRESSION_LEVEL