Delivering the first nonempty response

[ziollek]

Description

The aim of this issue is to provide a configuration option that slightly changes gathering response behavior.
When a new option is set final response to the client is prepared immediately after receiving the first nonempty response from one forward zone without waiting for responses from other zones.
All other requests should be canceled.

Expected behavior

If the option is set client gets rewritten responses from the fastest forward zone.

[gfelixc]

@ziollek Can you assign me this?

[ziollek]

Sure @gfelixc , do you have any experience with coredns plugins? If not manual can give you some insights.