CLC-4057 Perform health check on background job processing

https://jira.ets.berkeley.edu/jira/browse/CLC-4057

Because there's a natural (usually small) lag between sending a message (which happens after the "cluster" check-in is written) and handling the message (which happens before the "node-id" check-in is written), a certain number of "LATE"s can be expected during the day. But you're right that "warn" is a safe-enough level -- keeps them out of the alert emails, anyway!