WsFed remote signout doesn't work with ADFS4

[Tratcher]

@brockallen reported that singout cleanup messages don't work with ADFS4 because you can't configure both CallbackPath and RemoteSignOutPath to the same value. We may have to redesign how these are routed in HandleRequestAsync.

"ADFS has an odd requirement that the redirect URL after SLO must be beneath the URL used for signin response (for example "/wsfed" for wreply and for redirect after signout would need to be "/wsfed/" -- notice extra slash)."

We'll need to set up a repro.