This is a registry of all subcontract ID dependent objects. This includes:
LocalClientRequestDispatcherFactory, ClientRequestDispatcher, ServerRequestDispatcher, and
ObjectAdapterFactory.
XXX Should the registerXXX methods take an scid or not? I think we
want to do this so that the same instance can be shared across multiple
scids (and this is already true for ObjectAdapterFactory and LocalClientRequestDispatcherFactory),
but this will require some changes for ClientRequestDispatcher and ServerRequestDispatcher.
