dnsdist: Server pools are no longer ref counted

dnsdist: Server pools are no longer ref counted

Since the refactoring of the runtime configuration, the content of
a Server Pool is now in effect immutable, we have to create a new
copy and update it, so we no longer have to lock and reference count
Server Pools and their content.

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