Split unique values (groups) into shards according to the configured concurrency
during processing of the matching rows if the number of unique values exceeds the hardcoded threshold.
Previously this splitting was performed unconditionally at the merge stage when merging independently
calculated per-CPU states into a single state. It is faster to perform the split during rows processing
if the number of unique values is big.
This gives up to 30% perfromance improvements when these pipes are applied to big number of unique values (groups).
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