introduce external processing in indexer

Indexing is now split into three parts: first a preparation step
that collects the necessary information from the database and
returns it to Python. In a second step the data is transformed
within Python as necessary and then returned to the database
through the usual UPDATE which now not only sets the indexed_status
but also other fields. The third step comprises the address
computation which is still done inside the update trigger in
the database.

The second processing step doesn't do anything useful yet.

nominatim/indexer/indexer.py

@@ -187,14 +187,14 @@ class Indexer:
 
                     with WorkerPool(self.dsn, self.num_threads) as pool:
                         while True:
-                            places = [p[0] for p in cur.fetchmany(batch)]
+                            places = [p for p in cur.fetchmany(batch)]
                             if not places:
                                 break
 
                             LOG.debug("Processing places: %s", str(places))
                             worker = pool.next_free_worker()
 
-                            worker.perform(runner.sql_index_place(places))
+                            runner.index_places(worker, places)
                             progress.add(len(places))
 
                         pool.finish_all()