The systemd extender is a scheduled execution that ensures we
repeatedly let systemd know during startup that we are still starting
up. We cancel this scheduled execution once the node has successfully
started up. This extender is wrapped in a set once, which we expose
directly. This commit addresses this by putting the extender behind a
getter, which hides the implementation detail that the extener is
wrapped in a set once. This cleans up some issues in tests, that
ensures we are not making assertions about the set once, but instead
about the extender.
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