Add endpoints for healthcheck readiness and liveness probes

Add "/healthcheck/ready" endpoint for determining application readiness, i.e. the application has started up and is ready to receive requests by being brought into a load balancer.

Add "/healthcheck/live" endpoint for determining application liveness, i.e. the application is responding and is in a healthy, live state. Failing this test means the application has severe issues and might need to be restarted to become healthy again.

Basic load balancers with a single probe would use the "/healthcheck/ready" endpoint, more complex load balancers with separate readiness and liveness probes such as those in Kubernetes would use both the "/healthcheck/ready" and "/healthcheck/live" endpoints.