[Benchmark] Make TermsAggregationSearchBenchmark fairer to uninverted field data.

The benchmark indexes 200 unique full-width longs. For uninverted field data
we try to use the most memory-efficient storage, and in that case it would use
two arrays: one for the doc->ordinals mapping and one for the ordinal->value
mapping. Which is slower than what doc values do by storing directly the
mapping from docs to values.