For successful HTTP calls there were cases where no 'rpki_refresh' metric was
exposed, as FetchFile() return'ed without the bool return value set to true.
As the bool return value mainly seems to indicate that a file was successfully
fetch from an HTTP URL, the same behavior can be achieve by using the HTTP
status code to expose the metric.
This also contains some drive-by clean-ups.
Signed-off-by: Maximilian Wilhelm <maximilian@cloudflare.com>
