There were a couple of issues with unregistering nested records:
1) SC.ChildArray did not unregister nested records at all, so toMany
relations with nested records did not work properly.
2) When a child was unregistered from its parent, that did not propagate
to children of the child, leading to problems when nesting was several
3) There were two additional caches that were not being cleared when the
nested record was unregistered.
The previous commit incorrectly calculated which records to unregister
and register. All items after the passed in index need to be unregistered,
then all the new items should be registered, followed by the existing items
that were previously unregistered.