Rather than overloading InstanceDiff with a "Stub" attribute that is
going to be largely meaningless, we are just going to skip
pre/post-diff hooks altogether. This is under the notion that we will
eventually not need to "stub" a diff for scale-out, stateless nodes on
refresh at all, so diff behaviour won't be necessary at that point, so
we should not assume that hooks will run at this stage anyway.
Also as part of this removed the CountHook test that is now failing
because CountHook is out of scope of the new behaviour.
5654a676d9