ENH: Add new order statuses for broker integration

In talking to our colleagues, I've been convinced of two things:
- Naming this `broker_status` is unnecessary, as these are indeed legitimate statuses that aren't necessarily only applicable to broker-backed trading.
- Since these are legitimate statuses, and since we do want to make the transition from backtesting to live trading as smooth as possible, we should expose them just as we would any other order status (i.e. no need to coalesce them into the original open/filled/cancelled equivalent statuses)

To that end, I think what we should do is rename `self.broker_status` to `self._status` and keep access to it via the `.status` property. We should then expose a `reject()` method on the order (rather than extending cancel).