IB: brokercommand to cancel **all** orders.

Can there be a way to cancel all open orders with a new brokercommand? We can call it DO_CANCEL_ALL.

Specifically in IB, this can be done with the reqGlobalCancel function:
https://interactivebrokers.github.io/tws-api/cancel_order.html
https://interactivebrokers.github.i...t.html#a66ad7a4820c5be21ebde521d59a50053

The purpose of this is to keep tight controls on runaway orders for position tracking.

That would be DO_CANCEL,0. We'll implemet it on the next review of the IB plugin.