…nc (elastic#50244)

If a processor executes asynchronously and the ingest simulate api simulates with
multiple documents then the order of the documents in the response may not match
the order of the documents in the request.

Alexander Reelsen discovered this issue with the enrich processor with the following reproduction:

```
PUT cities/_doc/munich
{"zip":"80331","city":"Munich"}

PUT cities/_doc/berlin
{"zip":"10965","city":"Berlin"}

PUT /_enrich/policy/zip-policy
{
  "match": {
    "indices": "cities",
    "match_field": "zip",
    "enrich_fields": [ "city" ]
  }
}

POST /_enrich/policy/zip-policy/_execute

GET _cat/indices/.enrich-*

POST /_ingest/pipeline/_simulate
{
  "pipeline": {
  "processors" : [
    {
      "enrich" : {
        "policy_name": "zip-policy",
        "field" : "zip",
        "target_field": "city",
        "max_matches": "1"
      }
    }
  ]
  },
  "docs": [
    { "_id": "first", "_source" : { "zip" : "80331" } } ,
    { "_id": "second", "_source" : { "zip" : "50667" } }
  ]
}
```