stats_archives.json can be generated as invalid JSON

There have been examples where stats_archives.json has been generated as invalid json, this means it is not usable to parse out stats.

Specifically, this is because extra brackets are added between the different 'zooms', for example:

  {

    "year": {

      "status": "ok",

      "samples": [

      ...

     ]

    }

  },

  {

    "minute": {

      "status": "ok",

      "samples": [

...