dnsdist: Fix the number of concurrent queries on a backend TCP conn

dnsdist: Fix the number of concurrent queries on a backend TCP conn

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.

(cherry picked from commit 2848406e55b5a1d2e35bfa9f7c4dbb0c49989a1b)