Updates must be connected to a space event #69
Description
This is important to ensure that the client has all the latests keys before decrypting it.
In addition we want to ensure that the server can't inject an update from an already remove member as a new update. To do this new encrypted updates can only reference the current or newer space events. Using the clock and the references we can check for this rule.
The information should be stored in the AAD (additional authenticated data) of the AEAD ciphertext. This way also the server can verify it's correctness and reject old updates in case a newer space event already exist.