Adapt the strategy.shiftWalls() method

[Desrever Nu]

The shiftwalls method needs to be adapted for the modified order book. We will have a number of orders to shift at every price trigger now.

[Desrever Nu]

Deleting all orders can take up to half a minute (25 orders on bter) .

The new shift strategy could delete one order at the time and replace it right away at the new price . (mind wall collisions)