Clarify /system/state/boot-time and /system/state/up-time behavior in HA scenarios

[LimeHat]

Current definitions do not specify the exact behavior in HA scenarios (control module switchover). https://github.com/openconfig/public/blob/785b890a6369901d5cc227f1a86de98b694305d7/release/models/system/openconfig-system.yang#L449-L465

Is the expectation that a) values are preserved during an HA event (control module switchover), or b) values are reset during an HA event? - if the values are reset, are they reset to the boot time of the control module, or to the time of the switchover event?

cc @dplore @robshakir