dnsdist: Fix setting meta keys on response, pass them from question to response

dnsdist: Fix setting meta keys on response, pass them from question to response

This commit fixes setting Protocol Buffer meta keys on DNS response via Lua FFI:
the existing code was assuming it was possible to use the question methods on a
response object which is not true and would likely have ended in a crash at some
point.
It also propates meta keys set on a DNS question to the corresponding DNS response.
Before this commit the values were not passed along to the response which was quite
unexpected, especially for self-answered responses.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>