Refactor zone stub, refresh and notify event callbacks
requested to merge 3084-dont-schedule-zone-refresh-when-shutting-down into 3086-remove-workaround-for-server-mishandling-notify-with-soa-record-in-answer-section
This commit cleanups the way how we handle the revent->result in the stub_callback(), refresh_callback() and notify_done() zone callbacks.
Closes #3084 (closed)