CLC-2512 Is-updated feed with clustered warmup

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-2512

/api/my/updated_feeds triggers a warmup of merged feeds, and also returns a list of feeds and their last known hash and update timestamp. Warmups are rate-limited to 1 every 2 minutes. The UI can request updated_feeds as often as it wants and it will always be fast. 

There are some cleanups in cache_tools and HotPlate relating to the fact that we're preserving the old refresh API and its expire-everything approach alongside this new expire-only-merged-models approach. 

Carefully examine the cache expiration changes in settings.yml ; some of those values may need adjustment. We also may need to consider adding caching to some of the base proxies that don't have it yet.