Fixes potential reordering issue in extended lists.  Previous code
could flush a change to hardware when a former read lock holder or
write lock holder that had already been canceled and removed from the
list of an AtomicObject's readers/writers called backout.  (Which it
might, even if it had already been removed because of asynchrony in an
event's backing out and its calling backout on its touched objects.)
This commit takes care of that issue.