Fix & optimize incorrect function declarations (#918)

* All functions that access database must be declared as `STABLE`, not `IMMUTABLE` -- because database can change at any moment, e.g. during an update
* there are a few functions that could be made `STRICT` -- passing `NULL` as a parameter will always result in a `NULL`, but for some reason that causes a significant decrease in perf.
* tagged one function as parallel safe

NOTE: somehow `ST_AsMVT()` method of tile generation is showing 70-90% slowdown with this patch. I am not sure of why this is happening. If the reason is the `IMMUTABLE` -> `STABLE` change, we may have to dig deeper into PG optimization

layers/place/layer.sql

@@ -142,5 +142,6 @@ FROM (
          FROM layer_city(bbox, zoom_level, pixel_width)
          ORDER BY "rank" ASC
      ) AS place_all
-$$ LANGUAGE SQL IMMUTABLE
+$$ LANGUAGE SQL STABLE
                 PARALLEL SAFE;
+-- TODO: Check if the above can be made STRICT -- i.e. if pixel_width could be NULL