Draft: Making similar changes as in master-Powheg-FixMulticore which fixes multicore in master
I'm making similar changes in 21.6 as are done in !52040 (merged) for master, for the sake of consistency. Without these changes everything still work in 21.6, but not in master in multicore mode.
No need to sweep to master eventually.
Marking it as Draft at the moment, to get some feedback.