[FEATURE] Chirpstack v4.7.0 compatibility (SKFs stored in postgres now)

[vkynchev]

Hi @disk91, today I've updated to chirpstack 4.7.0 and noticed they've updated how they store the SKFs. The redis storage is now obsolete (which is awesome since redis is not made to persistently store important data). I'd love to help with that if you don't mind 🙂

Link to Chirpstack's changelog: https://www.chirpstack.io/docs/chirpstack/changelog.html#device-session-migration

Reference inside the source code for an example code that'll cause issues: https://github.com/disk91/helium-chirpstack-community/blob/30f870bcebee2c3ddce5babf8988af36ec39235e/src/main/java/eu/heliumiot/console/service/NovaService.java#L473-L480

PS: as of now I've resolved it temporarily by setting "ignore_empty_skf": false in the helium route, but I don't like this as a permanent solution...