dnsdist: Explicitly update the configuration thread-local copy

dnsdist: Explicitly update the configuration thread-local copy

This commits ensures that all DNSdist threads are regularly checking
if there is a new version of the runtime-modifiable configuration and
update their local copy if necessary. Regular accesses to the copy
are now fully read-only, meaning they do not invalidate the current copy.
It prevents the case where a function is invalidating the copy that the
caller is holding, and makes accessing the configuration cheaper.

Signed-off-by: Remi Gacogne <remi.gacogne@powerdns.com>
(cherry picked from commit c08ba9f1f2fe743c3e05313b2c329446e2e2d1b1)