Extend the Isabelle proof to cover the cases with stale balance proofs

[pirapira]

The Isabelle proof added in #206 only talks about how the settlement algorithm works with the up-to-date balance proofs.

For older balance proofs, it's worthwhile proving that the party who submits an older balance proof can only lose tokens.

[pirapira]

@loredanacirstea 's comment #206 (comment) on what has to be done.