Incorrect information on state channel tree data persistance

[nikita-fuchs]

In the Aeternity node implementation, the state trees are cached inside the node. Note that if the node restarts, cached data is not likely to survive (it is not persisted on each update for performance reasons.) in https://github.com/aeternity/protocol/blob/master/channels/OFF-CHAIN.md#channel_reestablish - according to Dimitar, this is not correct and state channel tree data would be stored permanently.