PLATFORM-292: WikiaNewtalk - improve memcache key structure

Improve the structure of the memcache key

Instead of ``wikia:shared_newtalk:<user ID>:My_Long_UserName`` use the following:

```
wikicities:WikiaSharedNewTalk:<anon's IP>
wikicities:WikiaSharedNewTalk:<logged-in user ID>
```

* code cleanup
* removal of c&p code that generated memcache keys

@wladekb / @michalroszka / @jcellary