Composable API handler, support v1 and v2 login.

Some refactoring & restructuring of srvRoot to support the existing Admin.Login API as well as allowing us to introduce a v2 Login API, to be proposed in a followup pull request.
If I understand you're changes correctly, this is no longer AnonRoot, this is the "always this root forever and always", and it is the rpc.MethodFinder that determines whether this is currently the Anon API or the active API.
If we want to go this route, I *definitely* recommend changing the name, because otherwise it is quite confusing.