[osg-submissions] Use atomic reference counts

[Robert Osfield]

Hi Mathias,

I have now done a more thorough review of your changes to Referenced
and now understand much better the relationship with
OpenThreads::AtomicPtr and how the observers get managed.  With this
better understanding I'm now happy with the code changes and have gone
ahead and merged, tested and submitted the changes.

Many thanks for you efforts and patience of atomic ref counting,
finally it's now all safely installed and purring along nicely :-)

Next up it's finding out how the rest of platform cope with the new
OpenThreads::Atmoic, fingers crossed things will port just fine.

Cheers,
Robert.