Bytecode / contract verification

[Eknir]

Dependent on: ethersphere/swap-swear-and-swindle#71

This is related to the chequebook payment module implementation:
After we connect and decide to use the chequebook as a payment module implementation, we should verify whether the bytecode at the counterparty's cheqeubook address matches with the expected bytecode of the chequebook implementation.
Checking only the bytecode is not enough: we should verify the content of relevant state variables, as these can be set during deployment to a value which prevents us from cashing out our cheques.

Example: if cheque.paidOut is set to MAX_UINT, no cheque can ever be cashed out.

Possible solutions are:

• Reading the value of relevant state variables at handshake and make sure they are as expected.
• Making an on-chain factory which deploys smart-contracts and keeps track of deployed smart-contracts. Hard-code the address of the factory in the code and at handShake verify whether the chequebook contract is deployed by the factory.

[ralph-pichler]

same as #1583?

[Eknir]

Quoting a conversation from: https://github.com/ethersphere/swarm/pull/1637/files

I don't really object, but if you remove a TODO it must be clear that it will be done later and not be forgot. So if for you it is clear that this is part of #1641, I am fine removing it. Otherwise make an additional note in the issue.

BTW. It's not necessarily a bad practice to leave a couple of TODOs in the code.

This one should actually be changed: it is already a package level function, it's the SimpleSwapBin argument which is missing.

Related to the TODO which was deleted: // TODO: have this as a package level function and pass the SimpleSwapBin as argument