Log out of single session, not all of them.

If a user has multiple SSO sessions using multiple devices and/or
browsers and press Logout in one of them, it seems most sensible to
log the user out of that particular session with the IdP - not all
of them.
Add XXX for SSOSessionCache.

I don't think the SSOSessionCache supports more than one concurrent
SSO session for a single user, but I don't think it is worthwhile
fixing since eduID will be using the MongoDB session cache anyways.
Clean up a lot of unnecessary _ variable prefixes.
Remove the hard coded usernames and passwords.

Regular eduID login infrastructure is working now, so the development
accounts can easily be removed.