/* Session transition */
Each confirmed participant (user_in_room_state == CURRENT_USER) keeps to indicators:
The active_session is the only session which is in IN_SESSION state. A session state moves to IN_SESSION when the user receives session confirmation from all participants. In that case the previously active session state will be DEAD.
When a new session get activated, all STALE sessions gets refreshed. That is, all sessions which were requested to be created (due to join or leave) during the creation of the newly active session, will be recreated according to the new participant list. Further more, the next_in_activation_line also points to the newly activated session.
Joining user (user_in_room_state == JOINING) does not keep any indicator.