collectChecks may run the same checks more than once

[cristiangraz]

In setting up this library, I initially added a database check to both AddLivenessCheck and AddReadinessCheck. In looking at handlers.go, it looks like the ready handler will run both liveness and readiness checks. I didn't see this documented anywhere, so I assumed each only ran their own specific checks as added.

Would it be possible to either 1) document the behavior in the godoc and README, or 2) treat each as separate checks. Maybe expose a third more explicit method AddLivenessReadinessCheck() if someone wants to add to both.

If both checks are going to be combined, it might make sense to treat the name as a unique identifier and ensure the same check (which may be registered in both places) is not run more than once. Otherwise the ready checks may call (for example) db.PingContext() twice instead of once for a single /ready check from Kubernetes.