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.

lib-sql/indices.sql

@@ -82,4 +82,14 @@ CREATE INDEX IF NOT EXISTS idx_postcode_postcode
       INCLUDE (startnumber, endnumber) {{db.tablespace.search_index}}
       WHERE startnumber is not null;
   {% endif %}
+---
+-- Table needed for running updates with osm2pgsql on place.
+  CREATE TABLE IF NOT EXISTS place_to_be_deleted (
+    osm_type CHAR(1),
+    osm_id BIGINT,
+    class TEXT,
+    type TEXT,
+    deferred BOOLEAN
+   );
+
 {% endif %}