Cexio client fix of "ticker" subscription + implementation of "l2update"

[maxi-rh]

I've noticed that ccxws subscribes to the event that responds with "tick" event which only gives a pair and the price without the volume. I believe this is not really useful so I added "ticker" request instead which returns useful information similar to other exchanges.

There is no "l2update" subscription either, but cexio allows to subscribe to "order-book-subscribe" event which then responds with "md-update" event, which includes order book details basically.

[maxi-rh]

Please check my variant of this implementation: #311