vtkAbstractTransform: move private members in internal class

Having std::mutex in the header creates the following issue when compiling under MS .NET

fatal error C1189: #error: <mutex> is not supported when compiling with /clr or /clr:pure.

Moving the private members internally resolves the issue.

see https://discourse.vtk.org/t/std-mutex-incompatible-with-c-cli/11050/2