JOJO - Ⅲ

In the contract `JOJOOperation` the role `_owner` has authority over the functions shown in the diagram below. Any compromise to the `_owner` account may allow the hacker to take advantage of this authority and extract all the funds via setting of bad `RiskParams` (fake Oracle, unexpected `liquidationThreshold` and `insuranceFeeRate`) . 

`validOrderSender` can "replay" a cancelled off-chain order or fulfill makers' orders in wrong order. `validOrderSender` prepares a set of orders to execute the `Perpetual.trade()`. Each order should be signed by `order.signer` or by her operator. However, the cancelled order can still be used until it expires. `nonce` part of the order is only used to distinguish orders, it is never checked on-chain and not "consumed" after cancellation. `validOrderSender` can use the taker order and fulfill her own maker order with the worst possible price, ignoring all the other maker orders with better prices.

Function `setPerpRiskParams()` allows to deregister the Perpetual from the Dealer even if there are open positions. Users with positive `credit` will suffer loss, users with negative will gain profit. It would be more fair to forcefully close all the positions before deregistering.