Ticket 16311

- Provide a hook into ``admin.filter.RelatedFieldListFilter`` to change the related objects to display.
- Implements ``RelatedOnlyFieldListFilter``which use that hook to limit the related objects to the ones found into the relation.
Yes, the ``test_relatedonlyfieldlistfilter_foreignkey`` test fails with your code:

```
- [(1, u'alfred'), (1, u'alfred'), (2, u'bob')]
?                  ----------------

+ [(1, u'alfred'), (2, u'bob')]
```
About performances. Well, it is not that bad. 
[Here](http://pastebin.com/vRRW6uZM) a copy//paste from debug toolbar SQL report from a project of mine with huge filters (request / time in ms).
This gives a rough idea.