Reduce compute effort of liboverrides resync process by only re-syncing
the parts of the override hierarchy that actually need it.
The main change compared to existing code (which was systematically resyncing
a whole override hierarchy), is that resyncing now operates over several
sub-hierarchies at once, each defined by their own 'resync root' ID.
This ensures that we do not get several new overrides for the same data inside
of the same hierarchy.
Implements T95682.
Differential Revision: https://developer.blender.org/D14079
1695d38989