TestCacheItem.test_item_state_transitions non-deterministic

[Jajcus]

While minor, I have seen this test fail by "skipping" intermediate cache states which are expected to be visited during the test. To make the test completely reliable, we basically need to know the last "update time" for the cache item (expose the 'now' obtained in CacheItem.update_state), and make sure the clock difference corresponds to the correct cache state.
I'm attaching a sample diff for a new test and CacheItem which exposes the last cache update time (although we probably shouldn't change the internals just to get the test to pass reliably).