dnsdist: Prevent dangling references in structured logging calls

dnsdist: Prevent dangling references in structured logging calls

I had not realized before that the `Logging::Loggable` ctor takes
a `const` reference to the object we pass to it, meaning that we
cannot pass temporaries to it without creating dangling references.

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