Enable optimizer_use_gpdb_allocators guc by default (#8771)

This guc makes ORCA use gpdb allocators. This allows for faster
optimization due to less overhead, reduced memory during optimization
due to smaller/fewer headers, and makes ORCA observe vmem limits instead
of crashing.
Authored-by: NChris Hajas <chajas@pivotal.io>

src/backend/utils/misc/guc_gp.c

@@ -2799,7 +2799,7 @@ struct config_bool ConfigureNamesBool_gp[] =
 			GUC_NO_SHOW_ALL | GUC_NOT_IN_SAMPLE
 		},
 		&optimizer_use_gpdb_allocators,
-		false,
+		true,
 		NULL, NULL, NULL
 	},