liveness (network) probes

[miekg]

Implement liveness probes: httpGet and tcpSocket. Execing a command in the container is hard(er), as we may need to get into the namespace - this is doable as we know the volumes that exist. But we should guard against an rm -rf / command as we run as root...
httpGet and tcpSocket are much less destructive and can be simply built into systemk

[pires]

Doing this should also cover readiness and startup probes.

[miekg]

link: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/
(note the named port feature)

[miekg]

See #38 for the probe's impl. Stil requires wiring up.