Port authentication handlers to new AuthenticationHandler interface.

1. Port all existing AuthenticationHandler components to new AH interface. This afforded some opportunity for relatively minor refactoring.
2. Move CAS 3.0 AH interface, related 3.0 components, and LegacyAuthenticationHandlerAdapter to legacy module to provide transition path for third-party authentication handlers.
3. Remove CAS 2.x transitional components from legacy module.
4. Remove deprecated authentication-related components.