Refactor the FakeNFS support into a TransportDecorator. * Move the existing boilerplate ReadOnly decorator logic in to a base class 'bzrlib.transport.decorator.TransportDecorator.' * Do the same to the ReadOnlyServer to create 'bzrlib.transport.decorator.DecoratorServer'. * Use the new decorator support to create a trivial FakeNFSTransportDecorator class in bzrlib.transport.fakenfs.