upstream: prioritize the reuse of connections instead of establishing new ones

[lecaros]

Currently, we're prioritizing the creation of new connections if there's no limit on them or if we are within the limits. This commit prioritizes the reuse of connections and only creates one if there are no available ones and we're within the limits (if any). This should address #10342 This also fixes a regression introduced in #10423 where we were not reusing connections in the av_queue.

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