[render_expired] Set back date by exactly 20 years for touch
 based expiry

With a set back of 20 years rather than to a specific date,
the actual render time gets preserved and can be checked in
e.g. mod_tiles tile status



git-svn-id: http://svn.openstreetmap.org/applications/utils/mod_tile@29197 b9d5c4c9-76e1-0310-9c85-f3177eceb1e4