Refactored the osg::Referenced observerset code so that it now uses a dedicated ObserverSet class,

which utilises a global recursive mutex that is dedicated to manage Observer and ObserverSet.

The new global mutex for observers avoids problems with deadlocks that were occurring previously when
an osg::Refenced object was being deleted at the same time as on osg::ObserverNodePath.

include/osg/ObserverNodePath

@@ -67,7 +67,6 @@ class OSG_EXPORT ObserverNodePath : public osg::Observer
 
         virtual bool objectUnreferenced(void* ptr);
 
-        mutable OpenThreads::Mutex      _mutex;
         osg::NodePath                   _nodePath;
         bool                            _valid;
 };