Fix EIP712 chain ID handling

[oberstet]

EIP712 contains the chain ID in signed typed data to prohibit cross-chain replay attacks.

We should run-time read the chain ID from within Solidity in the channel contract - currently, its hard-coded:

https://github.com/crossbario/xbr-protocol/blob/bf7edc0e45b180f88b4076774ce89f08f3709a92/contracts/XBRChannel.sol#L192

[oberstet]

• https://eips.ethereum.org/EIPS/eip-1344
• https://www.queryxchange.com/q/29_56749/retrieve-chain-id-of-the-executing-chain-from-a-solidity-contract/
• Add metamask_watchAsset MetaMask/metamask-extension#4606

[oberstet]

reading the chain ID from within a contract at run-time is not possible currently. it will be possible once this EIP is implemented https://eips.ethereum.org/EIPS/eip-1344

[oberstet]

ok, here is an update:

1. the new CHAINID opcode (EIP-1344) was deployed in Istanbul release
2. Solidity 0.5.12 exposes this opcode in assembly:

        uint256 chain_id;
        assembly {
            chain_id := chainid()
        }

• https://ethereum.stackexchange.com/questions/56749/retrieve-chain-id-of-the-executing-chain-from-a-solidity-contract/76764#76764
• https://solidity.readthedocs.io/en/v0.5.12/assembly.html#opcodes

[oberstet]

ganache CHAINID opcode currently is buggy: https://github.com/trufflesuite/ganache/issues/1643

[oberstet]

so above is now fixed. chain ID is dynamically read, as well as exposed on XBRNetwork:

    /// Used for EIP712 verification: network ID of the blockchain this contract is running on.
    uint256 public verifyingChain;

    /// Used for EIP712 verification: verifying contract address.
    address public verifyingContract;