`kraken` backend: `BrokerdStatus.remaining` should decrement by fill amount

[goodboy]

As per comment in response to question in #214 fill msgs should report how much of the original order size was cleared in the transaction.

We probably just need a little arithmetic to finish this but I don't want to delay that PR any longer as many other things need to be built around/on it.

[iamzoltan]

alright, I can tick this off next since it seems like it would be a quick PR.

[iamzoltan]

did this also get sorted?