Implement a proper backend

[kaiyou]

We need to write the data to a proper backend, ie. probably a SQL database given the current status.

The structure would be simple :

• instances, with latest sight date and version
• sightings with versions

[kaiyou]

Following the great job at migrating this to Go, let's try and implement a modular backend. Urgent backends that we would need to implement are:

• flat csv (current version for compatibility),
• elasticsearch/prometheus/telegraf (for more interesting statistics)