API isn't thread-safe

[stefanor]

The HealthCheck objects store state on themselves, which you need to check in subsequent is_ok calls.

Every run of the run() method first clears _ok and _details before populating them, 1 item at a time.

If there's a global HealthCheck object, shared between threads (sbbe has an example of such a thing), then is_ok could show you the empty status, at the start of another thread's run.

@yola/service-engineering.