[Data] Optimize sample_boundaries in SortTaskSpec

[z4y1b2]

Optimize sample_boundaries in SortTaskSpec to call numpy.quantile once to get all boundaries for each column.

This is much faster than the old impl when num_reducers is large (eg. 5000), because each time numpy.quantile is called it actually sorts the array and the old impl calls it num_reducers times for each column.

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[raulchen]

thanks!