revlog: drop the _generaldelta attribute from the imanifeststorage interface It never belonged here.

revlog: remove legacy usage of `_generaldelta` All core code is now getting the setting from the DeltaConfig object.

revlog: use the new Config classes in _testrevlog the mock object need to follow the new interface. We allow ourself a small hacky import since this is testing code. The legacy attribute are still here because some code still use them. We will drop them when this code is updated.