Various tweaks to make config work better for the router project.

- Reload now takes a callback, so we can better determine when reloading
  both files and mongo data has finished.
- When being used to load a single runtime config file, watch that file
  for changes, so the individual gatekeeper processes can more easily
  be notified of config changes while still running.
- Change 'reload' event to 'change' since this now seems more
  descriptive.
- Fix mongo connection backoff attempts continuing even after calling
  close() on the loader.