Feature request: Parallel upstream querying

[xopez]

Current behavior
When multiple upstreams are defined, ctrld queries the first one and only moves to the next if the previous upstream times out.
This means:

• No concurrency between upstreams.
• Failover introduces added latency (waiting for timeout).

Requested behavior
Add support for parallel upstream querying, where ctrld sends the DNS query to all configured upstreams at the same time at the listener policy and returns the first valid response it receives.

Benefits

• Lower latency: fastest upstream is used, no waiting for timeouts
• Improved reliability: protects against partial outages or degraded performance on some upstreams.
• Consistent behavior: avoids “stick” reliance on a single upstream.

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Edit:
The reason I want the feature is, that I want something like DoH and other protocols together so I don't really need to wait for the timeout for the other upstreams

[bouncy-brenda-beheaded-debra-petrov]

This is one feature that I do miss from when I was using the AdGuard app with Control D as its upstreams.