server: add a monitor endpoint

See https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-a-liveness-http-request

We add a new GET endpoint that the kubernetes cluster can ping and check for the liveness of the service.

We need to have a mutex in the watch function in case we get an error there. For the rest of the service, well, if the server crashed, the service will not answer and kubelet will know what to do.

Fixes #9 (closed)