Bugfix: make rollback to work.

Rollback did never worked right on composed handlers (handlers with
sub-handlers) because when the class attributes were unpickled, they were
bounded to a new object instance, and the sub-handlers references were
never updated (the sub-handler referenced directly the handler attributes).

Now all sub-handlers references the handler object itself (called parent)
so when the unpikle is done, all can reference to the same objects.