asynchrony support for BIND 9 query plugins
Currently, BIND 9's query plugin framework requires any hook works synchronously. But sometimes a hook action can be time consuming. Consider, for example, a hook that needs to send some kind of query (like a DB lookup) to an external backend server and waits for the response to complete the hook's action. Right now this has to be synchronous, blocking the BIND 9 worker thread handling the query, which also blocks subsequent DNS queries. It would be much better if it could be asynchronous, similar to the way BIND 9 handles recursive resolution from the query module.
Based on some experiments it doesn't seem to be difficult to extend it to support asynchronous hook actions. I plan to write a complete patch to implement the idea. This issue is a placeholder for that patch.