Review and changes from Jocelyn Fiat

NOTE: Now, I have a new interpretation of the "stale" flag.
Until now, I considered a nonce to be stale, if it is not the most current nonce of the user.
But I think a nonce is stale if it is EXPIRED, in the sense that it is too old.
This implies that, for each user, we only store the current nonce, and not a list.