EIP 712: use per-contract verifying addresses

[oberstet]

For all of the XBR contracts, the respective xyzFor() functions should use the respective contract instance address - not generally the address of the XBRNetwork contract.

Eg here https://github.com/crossbario/xbr-protocol/blob/0c29d5ec751cbde5db00ca37b703297adb0f8c16/contracts/XBRChannel.sol#L138

^ should use address(this) rather than market.network().verifyingContract()

[oberstet]

it's also important to stress: once we go to #9, above verifying address should be the addresses of the implementation contracts, rather than the proxy contracts.