[Data] Fix parallelism deriving heuristic to ensure parallelism stays w/in min/max bounds

[alexeykudinkin]

Why are these changes needed?

Currently, min/max parallelism isn't actually being enforced correctly -- for large enough clusters we will be scaling out too aggressively purely based on the # of available CPUs disregarding the target block-sizes.

This change

• Fixes parallelism detection heuristic to appropriately respect min/target block-sizes
• Makes block-sizes configs' defaults env-var-configurable
• Adjusts default min-block-size from 1Mb to 16Mb

Related issue number

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