If a consumer of the new `Authenticate` method gets passed a bogus token that's
a correctly-shaped UUID, it will correctly get an identity without a ACL
token. But most consumers will then panic when they consume this nil `ACLToken`
for authorization.
Because no API client should ever send a bogus auth token, update the
`Authenticate` method to create the identity with remote IP (for metrics
tracking) but also return an `ErrPermissionDenied`.
