Merge pull request #10156 from rgacogne/ddist-conn-reuse

dnsdist: Improve TCP connection reuse, add metrics

diff --cc pdns/dnsdist-console.cc
Simple merge

diff --cc pdns/dnsdist-tcp.cc
Simple merge

diff --cc pdns/dnsdist-web.cc
Simple merge

diff --cc pdns/dnsdist.hh
Simple merge

diff --cc pdns/dnsdistdist/dnsdist-tcp-downstream.cc
Simple merge

diff --cc regression-tests.dnsdist/test_API.py
Simple merge