#91 JOSSO logout should invalidate session cache

[Kurt Stam]

When logging out of JOSSO, the logout.do action invalidates the session, but the user's session may not be rechecked until the 'sessionAccessMinInterval' has
expired. The resulting behavior is that the user can click on a few more pages, before actually getting logged out.

A workaround is to reduce the sessionAccessMinInterval setting in the josso-agent-config.xml, but this increased the load on the SSO service.

The real solution should be to add a callback handle in the cache, so that the session cache can be invalidated in the logout.do action.