CLC-1992 Introducing StatsController to check up on HotPlate and JMSWorker

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-1992

Monitoring is dead-simple now that HotPlate and JMS are part of the app process. You just hit /api/stats and it gives you back JSON with relevant stats. I put Ruby mutexes around shared variables that might not be thread-safe.

I considered making the controller superuser-only but it's awfully useful from command lines and (maybe) monitoring scripts. Something to consider for later. I don't think it exposes any sensitive information.