Fallback strategy if fetching data from remote JSON endpoint has failed

[nroi]

When Flexo starts and the latency results are not available yet or obsolete, it fetches the JSON document from https://archlinux.org/mirrors/status/json/ and deserializes the JSON content. There are various reasons why this can fail:

• The ArchLinux server can be offline for maintenance.
• The URL could have changed (in fact, this has already happened in the past).
• The JSON format could have changed, for example, a field that used to be included in the past might have been removed.

To make Flexo more robust, we should include a fallback strategy to cope with this. Current plan:

• Include a new setting such as mirrors_auto.mirrors_status_json_endpoint_fallback.
• By default, this setting is set to something like file:///var/cache/flexo/state/mirrorlist_fallback.json (the nice thing is that we use CURL to fetch the JSON, and CURL understands file:// just as well as it does https://.
• Flexo updates this file (/var/cache/flexo/state/mirrorlist_fallback.json) as soon as it has successfully fetched, and deserialized, the mirrors from the remote endpoint. Make sure that the existing file is overwritten only after the deserialization has succeeded!
• When Flexo starts, and it has failed to fetch and deserialize the content from https://archlinux.org/mirrors/status/json/, then it will fetch the JSON from the fallback file.