swarm-bao-nats-callout-xkey runs on the queue's host before the
swarm-nats container, under the responder's store leaf: it runs
`swarm-nats-auth ensure-xkey`, reads the public half back from the
store, and writes it where the container's runtime config dir is. On a
change it restarts the server.
nats-server now always starts through the runtime wrapper, in manual
mode too. A privileged ExecStartPre renders auth_callout with the xkey
(and, in auto mode, the two generated nkeys) at every start, and fails
the server when no xkey has been fetched. The auto-mode key unit now
only mints seeds and writes their public halves.
The responder is wanted by nats.service, so a server started late, once
its key arrived, brings the responder up with it.
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