Code review comment for lp:~leonardr/launchpad/toggle-representation-cache

I need another review of the last couple revisions. While going through stub's review of my earlier branch, I remembered that we planned to set a default expiration time for items in the cache, so that when we tested on staging the cache wouldn't contain bad data indefinitely. I've implemented that code and a test, as well as responding to Jelmer's feedback and some more of Stuart's.

(Stuart's review is here: https://code.edge.launchpad.net/~leonardr/launchpad/test-representation-cache/+merge/26513)

I couldn't think of any way to test the expiration except from actually sleeping for 2 seconds, since I don't think we have a mocked-up memcached for use in testing. Suggestions are welcome.