Add `additional_hash` in the withdraw proofs

[pirapira]

The offchain withdraw proofs can be contained in a withdraw request message or in a withdraw message. In order to distinguish these two kinds in the signature checking, the signature checking must take cmdid or the additional_hash into account. https://github.com/raiden-network/raiden/pull/4284#discussion_r300417314

additional_hash is the future-proof approach (even when the withdraw-related messages contain more fields, the additional_hash will take care of all of them).

This issue keeps track of adding additional_hash in the signature checking of withdraw proofs. The result will look very similar to what's done on balance proofs.