OfflineSorter, which dimensional points uses heavily in the > 1D case,
works by reading one partition, a set of N unsorted values, from disk
and sorting it in memory and writing it out again.
The sort invokes a provided Comparator on two BytesRef values,
each of which is fully copied from the ByteBlockPool, when it could
often reference a slice from the pool instead.
Another byte copy happens when iterating through the sorted values.
This is an optimization ... I'm targeting 6.1.0 not 6.0.0!