Managing adresses

Adress are used a lot in the databases, they're present in at least dons, users and contreparties tables.

While it is important to enable users to specify a different adress for each contreparties (because they might want to offer them to someone, etc), it might be a bit more interesting to keep them in a separate table, to reduce data duplication.

We then must provide a way for a user to maintain their own adresses database.