Create a new finalization transaction to let validators withdraw some of their rewards

[Gnappuraz]

Validators will incur in substantial running costs and at least some of them will want to use the yield from the deposit to pay for them. In order to allow that we need to create a new transaction type that will let the validator withdraw some of the rewards obtained up to this point through the voting process. How much should be allowed to take is up for discussion because it will effectively diminish the amount of stake that would be slashed in case of misbehavior; so even though is necessary to let the validators collect without a logout that would lock their deposit for months, we need to carefully evaluate which portion of the inital deposit + rewards is safe to recover.