change updates to handle delete/insert workflow

This makes Nominatim compatible with osm2pgsql's default update
modus operandi of deleting and reinserting data. Deletes are diverted
into a TODO table instead of executing them. When data is reinserted,
the corresponding entry in the TODO table is deleted. After updates are
finished, the remaining entries in the TODO table are executed, doing
the same work as the delete trigger did before.

The new behaviour also works against the gazetteer output with its
insert-only mechanism.

test/bdd/steps/place_inserter.py

@@ -92,6 +92,12 @@ class PlaceColumn:
         else:
             self.columns[column] = {key: value}
 
+    def db_delete(self, cursor):
+        """ Issue a delete for the given OSM object.
+        """
+        cursor.execute('DELETE FROM place WHERE osm_type = %s and osm_id = %s',
+                       (self.columns['osm_type'] , self.columns['osm_id']))
+
     def db_insert(self, cursor):
         """ Insert the collected data into the database.
         """