ScatteredArcsORCGraph: Run webgraph sorters in their own pool

As a futile attempt to prevent ForkJoinPool from scheduling a new call to the same function in a thread where it is already running, which causes #4783 (closed).

Unfortunately, even calling .invoke() on a different pool seems to be enough to cause it to re-enter the function from the same thread.

Jenkins job DGRPH/gitlab-builds #593 succeeded .
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closed