Make ThreadPool's fields protected for derived access

Status Job ID Name Coverage
  External
passed cdash

passed ghostflow-branch-check-master

failed smtk@master-kerbin-windows-shared-release+examples+remus

00:02:34

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+private+remus

pending smtk@master-moho-linux-shared-relwithdebinfo+examples+python3+remus+vtk
pending smtk@master-moho-linux-shared-relwithdebinfo+examples+remus
failed smtk@master-moho-linux-shared-relwithdebinfo+examples+remus+vtk

00:09:48

passed smtk@master-norkit-osx-shared-release+examples+matplotlib

failed smtk@master-praxis-windows-shared-release

failed smtk@master-talosiv-osx-shared-release+examples+matplotlib

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib+private

00:10:31

passed cdash

failed smtk@master-moho-linux-shared-relwithdebinfo+examples+private+remus

00:11:10

passed smtk@master-norkit-osx-shared-release+examples+matplotlib

00:25:22

failed smtk@master-praxis-windows-shared-release

00:27:22

failed smtk@master-talosiv-osx-shared-release+examples+matplotlib

00:17:50

failed smtk@master-tatooine-osx-shared-release+examples+matplotlib

00:11:00