Change the cartodb_ids of aggregated overviews

Instead of arbitrary ids, the id of one of the aggregated records
is used, so that if it is used in the UI to query point
the information of one of the records grouped at the point will be
retrieved.

scripts-available/CDB_Overviews.sql

@@ -386,7 +386,7 @@ AS $$
              SUM(ST_Y(f.the_geom_webmercator)) AS sy,
              Floor(ST_X(f.the_geom_webmercator)/%2$s)::int AS gx,
              Floor(ST_Y(f.the_geom_webmercator)/%2$s)::int AS gy,
-             row_number() OVER () AS cartodb_id
+             MIN(cartodb_id) AS cartodb_id
           FROM %1$s f
           GROUP BY gx, gy
          )