add additional hook points, plus some minor refactoring

add additional hook points, plus some minor refactoring

- added some hook points that will be needed for a dns64 module later
- moved some code from the beginning of query_respond() to
  the end of query_prepresponse(); this has no effect on functionality
  but means we can have a hook point at the top of query_respond(),
  which seems nicer
- compressed duplicated code into query_zerottl_refetch() function
- added a qctx->answered flag so that a module can prevent
  query_addrrset() from being called from query_respond() when
  it's already been called from the module.