wait_for_login read the store leaf only while the agent waited for a login.
A logged-in agent now reads it every 5 minutes, and a paused one once before
it resumes, so a login made or deleted in the swarm UI reaches an agent that
never failed. A newer version is installed; a store that answers nothing
stored after a version was installed means the stored login was deleted, and
the agent is logged out as /api/logout does it, with the recorded version
forgotten so the next login installs. A read that fails changes nothing.
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