Support gRPC health checks

[johanandren]

gRPC protocol: https://github.com/grpc/grpc/blob/master/doc/health-checking.md

Now supported by k8: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-a-grpc-liveness-probe

An alternative could be that it would live in Akka gRPC and pick up the same health checks as management.