Fix Kraken WS orderbook updates and add checksum support

[jdx-john]

The existing Kraken implementation for order-book updates is incorrect. It assumes each update will include either one "a" or one "b" block and that the message contains exactly 5 elements.
In most cases this is the case however the API can and sometimes does send both an "a" and "b" block in the same message. I have verified this myself with a live feed (examples are is also shown in the API docs) and in this case the behavior is faulty. The market symbol is read wrongly and the whole message will therefore get missed leading to order-book mismatch locally.

My change follows the spec more accurately and also adds support for Kraken's checksum property (https://docs.kraken.com/websockets/#book-checksum). I do not know if other exchanges provide this but it is vital to ensure your local order book is correct in case of missed messages. I have implemented the simplest possible API change for ExchangeOrderBook.Checksum which could be used by other exchange implementations if desired.

[jdx-john]

Assuming this gets accepted I'll add an implementation of the Kraken checksum as a utility method on ExchangeKrakenApi, linked to #624