core: deserialize soft-reboot shutdown timestamps into previous-* fields

core: deserialize soft-reboot shutdown timestamps into previous-* fields

Until now the SHUTDOWN_START/FINISH timestamps were carried across a
soft-reboot in the same fields, so afterwards they described the
previous boot rather than the current one, mixing current- and
previous-boot data in the same set. Mirror what is now done on kexec
and move them into the PREVIOUS_SHUTDOWN_* fields on deserialization.

Also reset local timestamps for events that rerun (eg: generators,
units loading, etc) on soft-reboot and switch-root.