HealthCheck parallelization

As per discussion about health check parallelization:
https://groups.google.com/d/topic/metrics-user/2MCMin1i7tU/discussion

It's a first refactoring to see if I should base my work on this. I didn't get the response on mailing list, so I'm creating this issue to get more visibility. I'm postponing any serious work on this until I get a green light (so I don't waste time on doing something that's going to be scraped).