Add guarantees for keys in the `BaseSynchronizedData`

[Adamantios]

trafficstars

At some point, it would be nice to start warning the devs if they try to update a value in the BaseSynchronizedData.

Imagine the following situation: some user has no idea of the framework's internals and decides to create a key for their FSM in the db called all_participants.

It might be even better if we expose a different object to the devs than the one used by the framework.