Messaging: Shared Key Expiration

[micahalcorn]

In order to support arbitration, we enable users to send their shared keys to arbitrators for message auditing. We need to handle expiration of those keys so that arbitrators cannot continue to eavesdrop on conversations that may arise involving the same participants. We need to also define the circumstances under which the key should expire. Should it happen automatically after the dispute is resolved? Should the participants explicitly revoke the arbitrator's access? Should a new shared key be generated each time one is sent to an arbitrator, effectively limiting the scope of the arbitrator's access to only messages that were sent prior to the grantor's explicit action?

😅 This is not a v1 priority. cc @wanderingstan