Periodic health check source could use health check source internally

[andybradshaw]

I've got a use case for a health check that I would like to have refresh periodically in some background thread, but have the actual endpoint use some cached value of this check. I saw that there's a periodic package that almost does what I want. I'm wondering if it would make sense to update this package to take a status.HealthCheckSource instead of the current poll function? This would give people control over the parameters fields of the health check result, which can be useful for supplementing the health state.

[gregakinman]

@andybradshaw I think the latest changes should get you what you want