AllWatcherNextResults API schema differs from response

[Huw Wilkins]

The AllWatcherNextResults Delta returned from AllModelWatcher and AllWatcher is defined in the schema.json as an object, but the API response is a tuple.

The schema.json has:
```
        "AllWatcherNextResults": {
          "type": "object",
          "properties": {
            "deltas": {
              "type": "array",
              "items": {
                "$ref": "#/definitions/Delta"
              }
            }
          },
          "additionalProperties": false,
          "required": ["deltas"]
        },
        "Delta": {
          "type": "object",
          "properties": {
            "entity": {
              "type": "object",
              "additionalProperties": true
            },
            "removed": {
              "type": "boolean"
            }
          },
          "additionalProperties": false,
          "required": ["removed", "entity"]
        }
      }
```

The actual response is a tuple in the format ["entity type string", "change type string", {delta object}] e.g.

```
{
  "request-id": 1,
  "response": {
    "deltas": [
      [
        "model",
        "change",
        {
          "model-uuid": "123",
          ...
        }
      ],
      ...
    ]
  }
}
```

This is seen in 3.x but is also the same for 2.9.

[Harry Pidcock (hpidcock)]

Unfortunately I think there is some special encoding going on for Delta, you can see it in MarshalJSON() in rpc/params/multiwatcher.go