feat(health): Beef up health-check routine

A call to /health will now attempt to pull stats for each
of the configured lists. If any of these calls fail, the
health-check will fail.

A failed health-check will result in a 500-level HTTP response
code and a plaintext body beginning with "FAIL".
Merge pull request #2 from bcwaldon/beefy-health-check

feat(health): Beef up health-check routine