Randomize hprobe.timer schedule

[phmccarty]

For a better spread of heartbeat probe records, it would be nice to randomize the schedule for hprobe.timer. Perhaps we could use a similar solution as implemented in swupd-update.timer.

[alexjch]

@phmccarty : I believe the way this is currently set it's fine, quoting systemd documentation "In order to optimally stretch timer events over a certain range of time, make sure to set RandomizedDelaySec= to a higher value, and AccuracySec=1us." So I don't think swupd is truly random, because is not going to be evenly distributed since AccuracySec is not low.

I think the randomness introduced by AccuracySec is enough (from documentation: "Within this time window, the expiry time will be placed at a host-specific, randomized, but stable position that is synchronized between all local timer units". Additionally this setup offers support for a design goal of telemetry quoting documentation "This is done in order to optimize power consumption to suppress unnecessary CPU wake-ups"

[alexjch]

@phmccarty: PR #178 adds a random 6 hours time window.