Database bulk loaders helpers do not support nested FK attributes

[Celso Providelo]

The existing bulk loader helpers from lp.services.database.bulk (load_related() and load_referencing()) only support direct FK attributes, i.e. load_related(Foo, rows, ['fooID']).

But sometimes it is necessary to load deeper related objects, i.e. load_related(Foo, rows, ['bar.baz.fooID'])

That can be achieved by replacing "map(partial(getattr, obj), fk)" by "map(attrgetter(fk), obj)" or something along this line.