rewrite address interpolation so that planet_osm_node is not needed

This allows address interpolations to work correctly when flatnode storage
is used for node coordinates.

To fix interpolations in an existing database, follow these steps:

  * invalidate all interpolations (in psql):
    `UPDATE placex SET indexed_status=2 WHERE rank_search = 28`
  * disable updates:
    ./utils/setup.php --create-functions --create-partition-functions
  * reindex the whole lot:
    ./utils/update.php --index --index-instances <number of your cpus>
  * enable updates again:
    ./utils/setup.php --create-functions --enable-diff-updates --create-partition-functions

sql/postgis_15_aux.sql

@@ -0,0 +1,14 @@
+-- Splits the line at the given point and returns the two parts
+-- in a multilinestring.
+CREATE OR REPLACE FUNCTION split_line_on_node(line GEOMETRY, point GEOMETRY)
+RETURNS GEOMETRY
+  AS $$
+DECLARE
+  frac FLOAT;
+BEGIN
+  frac := ST_Line_Locate_Point(line, point);
+  RETURN ST_Collect(ST_Line_Substring(line, 0, frac),
+         ST_Line_Substring(line, frac, 1));
+END
+$$
+LANGUAGE plpgsql;