Add support for the prefetch-key option.

* `prefetch-key`: \<yes or no\>:
              If yes, fetch the DNSKEYs earlier  in  the  validation  process,
              when  a  DS  record  is encountered.  This lowers the latency of
              requests.  It does use a little more CPU.  Also if the cache  is
              set to 0, it is no use. Default is no.