ENH: Optimized qMRMLSceneModel::nodeIndex performance

qMRMLSceneModel::nodeIndex is called tens of millions of times, therefore it's optimum performance is critical.
Find MRML nodes by object pointer instead of ID comparison to improve the performance.

Also, always do lazyupdate for qMRMLSceneModel if scene is closing to unsubscribe from all events at once the scene is closed.

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22666 3bd1e089-480b-0410-8dfb-8563597acbee