Ensure backrefs accommodate for op_bulk_replace

Ensure backrefs accommodate for op_bulk_replace

Fixed 1.2 regression regarding new bulk_replace event
where a backref would fail to remove an object from the
previous owner when a bulk-assignment assigned the
object to a new owner.

As this revisits the Event tokens associated with
AttributeImpl objects, remove the verbosity of the
"inline lazy init" pattern; the Event token is a simple
slotted object and should have minimal memory overhead.

Change-Id: Id188b4026fc2f3500186548008f4db8cdf7afecc
Fixes: #4171