Rework authentication and session management code:
- better tracking of JAAS Subject related properties (principals, public, and priviate credentials)
- better tracking for which authentication method is used for which principal
- no more reliance on the serlvet container session during any step of the authentication process
- raise an error when, during force authn, the authenticated principal is not the same as the previously authenticated principal - SIDP - 196
git-svn-id: https://subversion.switch.ch/svn/shibboleth/java-idp/branches/REL_2@2754
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