Make auth tickets expire when node goes offline

[MOZGIII]

We don't need to keep the auth ticket around when it's not used anymore by an online node. Keeping it in the system prevents us from forcing the re-authentications on node restarts and etc - which we want to do by design.

[MOZGIII]

We've discovered that using im_online currently is unfeasible, due to too tight coupling with the staking concepts. We'll tackle this later when we have more time.