- Responses received by the dispatch are no longer sent to the caller
  via a task event, but via a netmgr-style recv callback.  the 'action'
  parameter to dns_dispatch_addresponse() is now called 'response' and
  is called directly from udp_recv() or tcp_recv() when a valid response
  has been received.

- All references to isc_task and isc_taskmgr have been removed from
  dispatch functions.

- All references to dns_dispatchevent_t have been removed and the type
  has been deleted.

- Added a task to the resolver response context, to be used for fctx
  events.

- When the caller cancels an operation, the response handler will be
  called with ISC_R_CANCELED; it can abort immediately since the caller
  will presumably have taken care of cleanup already.

- Cleaned up attach/detach in resquery and request.