Have check-mirrors archive speed statistics
Currently, check-mirror has a `--minimum-speed` option that defines under which speed check-mirror returns the mirror as "slow" in the cron job log.
This makes those statistics hard to analyze and the logs that we receive longer and little informative. We should instead have check-mirror store those statistics, for example in a YAML file named according to the date and time. And only output in the logs the real errors.
git clone https://git.tails.boum.org/check-mirrors