Client: Add PARALLEL and VOLATILE categories

I'm not 100% sure what's appropiate for plproxy functions so, for now,
I've set them up as VOLATILE and PARALLEL UNSAFE

client/sql/10_geocoder_server_conn.sql

@@ -13,4 +13,4 @@ BEGIN
   SELECT trim(both '"' FROM db_connection_str) INTO db_connection_str;
   RETURN db_connection_str;
 END;
-$$ LANGUAGE 'plpgsql';
+$$ LANGUAGE 'plpgsql' STABLE PARALLEL SAFE;