* if a runner pod starts up with an invalid token, it will go in an
infinite retry loop, appearing as RUNNING from the outside
* normally, this error situation is detected because no corresponding
runner objects exists in GitHub and the pod will get removed after
registration timeout
* if the GitHub runner object already existed before - e.g. because a
finalizer was not properly run as part of a partial Kubernetes crash,
the runner will always stay in a running mode, even updating the
registration token will not kill the problematic pod
* introducing RunnerOffline exception that can be handled in runner
controller and replicaset controller
* as runners are offline when a pod is completed and marked for restart,
only do additional restart checks if no restart was already decided,
making code a bit cleaner and saving GitHub API calls after each job
completion
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