Merge pull request #15306 from rgacogne/ddist-exit-callbacks

dnsdist: Add support for calling Lua methods when exiting

diff --cc pdns/dnsdistdist/dnsdist-lua.cc
Simple merge

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

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