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Fix T39318: Blender 2.70 crash when I link an asset

Issue is a regression since threaded objetc update and caused
by the fact that some objects might share the same proxy object.

It's all fine but object_handle_update() will call update for
a proxy object which screws up threaded update.

The thing is, proxy object is marked as depending on a scene
object and such a call makes it so the children objetc is
being updated.

This is really bad and depsgraph is to take all responsibility
on updating the proxy objects.

So for now used a simple solution (which is safe to backport
to 'a') which is skipping proxy update if the scene update is
threaded and based on the DAG traversal.

There are some still areas which calls object update directly
and for that cases proxy object is still being updated from
object_handle_update().