prefill: deadlock issue

The https download of the (root) zone is blocking and it uses OS DNS. That combination will dead-lock e.g. in the case when kresd is the (only) resolver for the OS on which it runs. Originally discovered in #506 (closed).

Plan – implementation details: I expect we'd better convert the fetch to use lua-http library, as it's asynchronous and has a relatively convenient API for this.