Implement request cancellation

[puddly]

This will require a lot more plumbing for zigpy and ZHA to take advantage of properly and to actually be tested.

This PR provides a way to cancel requests that are still in a pending state and will (theoretically) avoid wasting airtime. A few caveats:

• Route discovery for all routing modes is a broadcast and cannot be cancelled. We can stop relaying our own copies of a broadcast but this likely won't do much once other routers on the network pick it up.
• Group commands are also broadcasts and cannot be cancelled.
• If we know a route to a device, cancellation would only help if Ziggurat is in a retry state. Otherwise, we send out the request instantly.

My eventual goal is to explore something like interaction_id for ZHA, where we track (in software) commands that affect an entity and cancel/replace pending ones that have been enqueued for set amount of time. For example, OnOff:off() enqueued after OnOff:on() should replace the on command. A more visible application would be dragging a slider to set the color of a bulb: instead of buffering a hundred lighting commands, we rate limit to one per second.