This adds a new `id` property to the ObjectStore.Object, that is
meant to reflect the identifer of the Stage to build the contents
of it. This will help to transparently access objects that have
been built but not committed to the store.
Setting the `base_id` of an object will also set its `id`. When
the object is then modified via write() the `id` will be set to
None, since no the content and the id are out of sync. In the
same way, restting an object will reset its `id` to None.
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