Enforce concurrent session limit in KnoxSSO

Once, KNOX-2777 is ready, the next step is to wire that verifier implementation into the KnoxSSO flow such as it throws an authorization error (FORBIDDEN; 403) when a user tries to log in to UIs (both Knox's own UIs or UIs proxied by Knox) but that user exceeds the configured concurrent session limit.

Basic logout handling should be covered too:

• manually clicking on the logout button
• subscribing to a session timeout event (you may want to talk to smore about this)