use session_user instead of current_user

scripts-available/CDB_Username.sql

@@ -1,13 +1,6 @@
--- Function returning the username of the provided user
-CREATE OR REPLACE FUNCTION _CDB_Username(pg_user TEXT)
-RETURNS text
-AS $$
-  SELECT CDB_Conf_GetConf(CONCAT('api_keys_', pg_user))->>'username';
-$$ LANGUAGE SQL STRICT IMMUTABLE PARALLEL SAFE SECURITY DEFINER;
-
--- Function returning the username of the current user
+-- Returns the cartodb username of the current PostgreSQL session
 CREATE OR REPLACE FUNCTION CDB_Username()
 RETURNS text
 AS $$
-  SELECT _CDB_Username(current_user);
-$$ LANGUAGE SQL STABLE PARALLEL SAFE;
+  SELECT CDB_Conf_GetConf(CONCAT('api_keys_', session_user))->>'username';
+$$ LANGUAGE SQL STABLE PARALLEL SAFE SECURITY DEFINER;