[osg-submissions] Use atomic reference counts

[Robert Osfield]

Hi Mathias,

I've have just done a trial integration of OpenThreads::Atomic into
osg::Referenced using your changes, and performance results are pretty
encouraging under Linux/Intel quad core - I've seen 2.8 to 3.8%
improvement in overall framerate on a large town model I have, this
for the whole frame so means that the actual improvement during
cull/draw is better than this percentage.

I won't commit the changes I've just merged though, as I recall from
my first pass review that the solution for the observers wasn't ideal,
so I'll go back and review these details and look for a better
solution right now.

Robert.