CLC-3960 Only cache a single copy of the Financials feed

https://jira.ets.berkeley.edu/jira/browse/CLC-3960

The CFV feed (which can be huge for students who have been here a while, and which is currently cached in three copies which vary only slightly) appears to be our biggest memory hog and so I focused on it first. As we go on to trim other cache, we might spot some patterns for re-use.
Fair enough, @ctweney . The method is called by the cache-expiring job to decide whether to expire an extra class+"JSON"+ID key along with the usual class+ID key. I'll find a different way to deal with that.