[RF] Use `UInt_t` for `_refCount` elements in RooSTLRefCountList
[RF] Use `UInt_t` for `_refCount` elements in RooSTLRefCountList

The `unsigned long` type should not be used for persistent data members
because its size is not guaranteed to be platform independent. The
`UInt64_t` type should be used instead, but actually it's not necessary
to have such large integers in a reference counter. Given that each
`RooAbsArg` has some data members that are `RooSTLRefCountLists`, we can
save some storage space by using the 32-bit unsigned integer type `UInt_t`.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root