Rewrite dns_resolver and dns_request to use netmgr timeouts

Rewrite dns_resolver and dns_request to use netmgr timeouts

- The `timeout_action` parameter to dns_dispatch_addresponse() been
  replaced with a netmgr callback that is called when a dispatch read
  times out.  this callback may optionally reset the read timer and
  resume reading.

- Added a function to convert isc_interval to milliseconds; this is used
  to translate fctx->interval into a value that can be passed to
  dns_dispatch_addresponse() as the timeout.

- Note that netmgr timeouts are accurate to the millisecond, so code to
  check whether a timeout has been reached cannot rely on microsecond
  accuracy.

- If serve-stale is configured, then a timeout received by the resolver
  may trigger it to return stale data, and then resume waiting for the
  read timeout. this is no longer based on a separate stale timer.

- The code for canceling requests in request.c has been altered so that
  it can run asynchronously.

- TCP timeout events apply to the dispatch, which may be shared by
  multiple queries.  since in the event of a timeout we have no query ID
  to use to identify the resp we wanted, we now just send the timeout to
  the oldest query that was pending.

- There was some additional refactoring in the resolver: combining
  fctx_join() and fctx_try_events() into one function to reduce code
  duplication, and using fixednames in fetchctx and fetchevent.

- Incidental fix: new_adbaddrinfo() can't return NULL anymore, so the
  code can be simplified.