LUCENE-8673 it was introduced radix partitioning for merging segments. It currently works the same when you have data offline and on-heap. It makes sense when data is on-heap, not to have multiple copies but perform the partitioning always in the same object, similar to what it is done with `MutablePointValues`.
This will allow to hold more points in memory as well because we don't have multiple copies of the same data as we recurse.
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