http.subscribe_api_call

[robnagler]

API calls come in two styles:

• call_api: send and wait for reply
• subscribe_api: return APISubscription, reply(), destroy (will unsubscribe/cancel).

Updates are replies that do not terminate a subscription. An APIError on a subscription automatically terminates the subscription.

@quest.publisher is a decorator on APIs that allow subscriptions, which run until completion (in new task): calling publish_reply for updates (intermediate replies) and returning the final reply (if ever).

I suspect we'll want to introduce a "channel" concept, which is a Queue that returns events to the publisher so that it can be canceled (Queue.shutdown(immediate=True) or a PriorityQueue where cancel is high priority item).