Secure json parsing

[delvedor]

This pull request adds a protection mechanism against malicious JSON documents, avoiding prototype pollution attacks.

Further reading: Square Brackets are the Enemy

[matAtWork]

This breaks perfectly valid existing code:

Consider the request:

POST /myindex/_search
{
  "size": 0,
  "aggs": {
    "constructor": {
      "terms": {
        "field": "anything",
      }
    }
  }
}

The response is:

{
  "took": 33,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 1,
      "relation": "eq"
    },
    "max_score": null,
    "hits": []
  },
  "aggregations": {
    "constructor": { /*** CAUSES AN ERROR FOR VALID REASON ***/
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": []
    }
  }
}

This will now throw a DeserializtionError on a perfectly valid response. It also occurs in many other contexts which depend on the data (such as mtermvectors) and not the request, making it impossible to avoid defensively.

[delvedor]

Hi @matAtWork, please see #1408 (comment).

[matAtWork]

Warning to other users: this PR introduces a breaking change that is dependent on the data in your ES index.

More details are in the above issue