SEC-2002: Added events to notify of session ID change

Session fixation protection, whether by clean new session or
migrated session, now publishes an event when a session is
migrated or its ID is changed. This enables application developers
to keep track of the session ID of a particular authentication
from the time the authentication is successful until the time
of logout. Previously this was not possible since session
migration changed the session ID and there was no way to
reliably detect that.

Revised changes per Rob Winch's suggestions.