Updates for snapshots_changed property

Currently, snapshots_changed property is stored in dd_props_zapobj, due
to which the property is assumed to be local. This causes a difference
in behavior with respect to other readonly properties.

This commit stores the snapshots_changed property in dd_object. Source
is not set to local in this case, which makes it consistent with other
readonly properties.

This commit also updates the date string format to include seconds.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #13785