Fix potential infinite loop in CDB_QueryStatements

This fix was already present at one point in cartodb/lib/sql
(where the code was copied from) but in a different branch than
the one the code was initially copied from.

The fix depends on plpython language which becomes a new dependency.

sql/test_setup.sql

@@ -1,5 +1,6 @@
 CREATE EXTENSION postgis;
 CREATE EXTENSION schema_triggers;
+CREATE EXTENSION plpythonu;
 CREATE EXTENSION cartodb;
 CREATE FUNCTION public.cdb_invalidate_varnish(table_name text)
 RETURNS void AS $$