CLC-1813 HotPlate fixes

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-1813

Significant rewrite here: HotPlate now warms up everything in 1 thread. This simplifies the programming and removes some issues with class resolution that I had. 

Also, we now write to the UserVisit table at most once every 4 hours. This should improve our load test performance. 

Finally, I noticed a number of proxy classes that weren't getting expired, and added them to the cache invalidator list.