Also multithread at the tree level and not below. Same fix for 4a93b9c5 as d74268d0
Reported in https://discourse.vtk.org/t/vtkhypertreegridevaluatecoarse-data-race-in-vtkthreadedtaskqueue-produces-short-output-array/16369/3
No new tests, because race condition is hard to trigger in testing
Backport: release