- Add /.cdn-util endpoint (protected by key)
- Add Prometheus metrics (disabled by default) under /.cdn-util
- Replace configuration loader with Viper, config files are now
  supported
- Rename some config flags + environment variables

- object.type == 1 now means "redirect"
- object.long_url is not object.dest_url
    - `ALTER TABLE objects RENAME COLUMN long_url TO dest_url;`
- ensure that 301 with `Location` header is returned on non-preview
  redirects
- set major version to 0, as breaking changes such as this can be
  introduced at any time