About 15% regression on string sorts

Post https://github.com/quickwit-oss/tantivy/pull/2726, there is about a 15% regression on string sorts.

I have not had time to triage this yet, but I strongly suspect that it is due to impl SortKeyComputer for SortByString doing individual lookups to the column's dictionary per term: https://github.com/quickwit-oss/tantivy/blob/d0e16001357b0238645d2e09db59e913b09fee07/src/collector/sort_key/sort_by_string.rs#L62-L72

When ordering by strings, the resulting values will be sequential in the column's dictionary. Because the dictionary is compressed, each of these lookups will decompress a block of the term dictionary, and since the values are potentially contiguous in the dictionary, this can mean that we decompress the same block multiple times.

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Previously on main, this used sorted_ords_to_term_cb to batch convert the TermOrdinals into terms. One way to get this performance back would be to change SegmentSortKeyComputer::convert_segment_sort_key into a batch method.