Better generalization for landuse=residential. (#1414)

This PR creates larger polygons of `residential` land-use areas for upper zoom levels (especially for zoom 6 - 10).

Clusters are created with the same logic as for [`building on z13`](b14da2eafb/layers/building/update_building.sql (L34-L49)).

The generalization of land-use prolongs the `import-sql` step of the `landuse` layer. But it is still faster than the `building z13` generalization which with the parallel importing keeps the whole `import-sql` time untouched.

layers/landuse/landuse.yaml

@@ -49,6 +49,7 @@ layer:
     query: (SELECT geometry, class FROM layer_landuse(!bbox!, z(!scale_denominator!))) AS t
 schema:
   - ./class.sql
+  - ./prep_landuse.sql
   - ./landuse.sql
 datasources:
   - type: imposm3