Fixed atomic int compile issues on Windows.

It turns out that forward declaring various atomic functions
(_InterlockedXXX) is not straightforward. I ran into situations
where this did not link. So I moved all of the atomic
implementation for Windows to the cxx file and properly
included vtkWindows.h. This should fix all compile/link issues
of atomic ints on Windows. Note that the main reason that I
chose not to include vtkWindows.h in the header is that
atomic ints are used by all subclasses of vtkObjectBase and
including vtkWindows.h all over the place would degrade
compile performance significantly.

Change-Id: I00bf173140587b07665d7d5d84ab0cad5bfda7fe
Merge topic 'atomic-compile-issue' into master

75062ae4 Fixed atomic int compile issues on Windows.