Saved JSON state can be corrupted

[Peter Wood]

For Any ZDA or ZStack saved configuration, it appears that it can be wiped out with a 0 byte JSON file. This seems to occur with no errors in logs - it only seems to happen during a shutdown. It's possible (given the frequency of saving) that a shutdown of the controller can result in corrupt configuration.

Consider approaches to avoid this:

• Shifting saving to some kind of safe goroutine.
• Save as file.new, rename file.json to file.old, rename file.new to file to json. rm file.old on completion (or keep history?)

[Peter Wood]

This is particularily bad if zstack is corrupted as the IEEE<->NetworkAddress mapping is lost. ZDA will at least rediscover and attempt to reenumerate (though likely to fail on fiddly end devices).

[Peter Wood]

This was addressed a with the recent mqtt merge, new file is saved out to a different file initially and then renamed. Old file is kept until process is completed.