NGC: make save format resilient to forwards compatibility issues

[iphydf]

E.g. by saving only the group key + password pairs in one array, and all the extra information in another array. This way we can more easily guarantee that the first array stays compatible while the second array could change format without breaking users. The second array would contain saved peers and other extra info that can be recovered, while the first one contains critical info that can't be recovered by connecting to the group.