Run dark cleaing loop in `pikerd` subactor

[goodboy]

As per the final unfinished item in #402, it's obviously ideal to be able to keep dark-cleared (aka algorithmic) order processing running when a client/UI is not connected to an emsd.

We need to persist both the rt quote feed and dark clearing task with any spawned brokerd order-dialog daemon-task.

Implementation should require a slight refactor to move components from _emsd_main() into Router.maybe_open_brokerd_trades_dialogue().

---

Testing:

• [x] user should be able to submit a dark order, detach the chart/client and see the order clear on the pikerd console output
• [x] when a dark order is cleared when no chart is attached the order line should be removed from view on re-connect
• [x] we may want to add notification support as is found in the OrderMode.on_exec() handler to the daemon such that triggered orders can cause a notification that use can then use as reason (or source) for starting a client/chart?

[goodboy]

I think this is done now right?

[goodboy]

Yeah pretty sure this landed in https://github.com/pikers/piker/pull/408

I think it would be good however if we could write some tests for these mostly simple cases and just do some state checking on the paper engine to verify all the bullets above?

I think i'll just make a new issue to cover the test suite requirements (ping @algorandpa)