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.
Ugh. This method is now deceptively named. The feed DOES cache json, but does it through the normal get_feed method instead of get_feed_as_json. That is going to cause astonishment later.