Rework authentication and session management code:

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 ab3bd59b-922f-494d-bb5f-6f0a3c29deca