Issue #552 Avoid usage count in SSO opt-out check.

Reorganize code to perform authentication beforehand and store reference
to successful Authentication event that can be consulted for SSO opt-out
check afterward.

Deals with #552 
I honestly don't know. I've used this style elsewhere [1] and it passed muster (build and review), so I assume it's ok.

[1] https://github.com/Jasig/cas/blob/d38d201a2c594e2bc656b2166a4918f7cdb5867d/cas-server-core/src/main/java/org/jasig/cas/authentication/AuthenticationBuilder.java#L272