Use logical cpu number as the default value of fine grained stream count

[windtalker]

Bug Report

Please answer these questions before submitting your issue. Thanks!
Currently, TiDB use physical cpu cores as the default value of fine grained stream count, this is due to TiFlash used to use physical cpu cores as its default max_threads.
From v7.5.0, TiFlash use logical cpu cores as the default value of max_threads, TiDB should update the default fine grained stream count to logical cpu cores as well

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3. What did you see instead (Required)

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