feat: added step to dog for LR != CR support

[hexonaut]

Not super sold on the name step, but basically this small addition will enable to set the liquidation ratio to be less than the vat collateralization ratio. If this has to be pushed after the initial LIQ-2.0 launch so be it, but I figured it was worth discussing to maybe sneak it in while we are still finalizing deployment.

The step multiplication is performed to the art instead of the art * rate so as to not to hit the overflow limit.

[godsflaw]

We think this is a good change, but it wasn't discussed in MIP45 and is a heavier change since the code freeze. There is a change to have an ilk based sin queue in #147 that will require us to update the vow and the dog in the future. We were thinking we should queue this up for that release.