Add a user-definable persistence backend

Add a persistence backend, which allows the user to define how will the persistence detail will be stored and recovered.
This will allow the user to use proprietary databases and storing services as the persistence backends.

This backend will be used, by registering callbacks for object's storing and retrieving. The persistence backend will call the user callbacks with a string-like object, and an id for the object being stored. To retrieve an object, the user's callback will get an id, and will return the string-like object stored before.
This mechanism will later be used to store and retrieve the logbook, the flow details, and the atom details.