Finish implementing session save/restore

[terrillmoore]

While studying #21, I discovered that I had forgotten to implement session restore. Just as well, as the session format I chose didn't represent all the needed information. But this needs to be finished.

[ThomasBlt]

@terrillmoore hasn't this been dealt with by the addition of GetSavedSessionInfo ?
If true what's the need for NetSaveFCntUp and NetSaveFCntDown as there is no restore function for them ?

[jangrewe]

Yes, please! I've made the first steps with LoRa on an ESP32, and this wrapper helped me a lot!
I've got most functionality working, now all i need is some way to save/restore the session info - but my coding skills are lacking, a lot, to be able to implement this.

[jdeus]

Still open ?

[jangrewe]

@jdeus have a look right below the title of this issue. if you see a green badge that says "(!) Open", then yes, this issue is still open.