* UptimeClock is a chrono-compatible seconds-precision clock.
* Like UptimeTimer, its purpose is to make it possible for clients
to query the uptime thousands of times per second without a
significant performance hit.
* UptimeClock decouples itself from LoadManager by managing its
own once-per-second update loop.
* Clients now traffic in chrono time_points and durations instead
of int.
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