Align the management of the state of transient and persisted representations

[sbegaudeau]

Once #3304 is done, we should be able to remove some of the differences in the way the state of a representation is managed between transient and persisted representation. Technically, the only different between the two should be:

• How to retrieve the initial state (computed on the fly vs retrieved from the database)
• What to do when a new state is computed (nothing vs persist it in the database)

By aligning the way the state is managed, we should be able to reduce the custom code that should be written for each representation, lower the risk for potential errors such as double saving and offer a uniform way to access the state of a representation.