dnsdist: Fix the number of concurrent queries on a backend TCP conn #11718
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Short description
When we are in the process of sending a query to the backend, that query is no longer accounted in the "queued" queries nor it is in the "queued" responses, but we need to take it into account.
Otherwise we might be sending two concurrent queries to a backend that does not support out-of-order processing (increasing our latency), or even worse to one that does not support pipelining.
Checklist
I have: