Aggregations: Return the sum of the doc counts of other buckets in terms aggregations.

This commit adds a new field to the response of the terms aggregation called
`sum_other_doc_count` which is equal to the sum of the doc counts of the buckets
that did not make it to the list of top buckets. It is typically useful to have
a sector called eg. `other` when using terms aggregations to build pie charts.

Example query and response:

```json
GET test/_search?search_type=count
{
  "aggs": {
    "colors": {
      "terms": {
        "field": "color",
        "size": 3
      }
    }
  }
}
```

```json
{
   [...],
   "aggregations": {
      "colors": {
         "doc_count_error_upper_bound": 0,
         "sum_other_doc_count": 4,
         "buckets": [
            {
               "key": "blue",
               "doc_count": 65
            },
            {
               "key": "red",
               "doc_count": 14
            },
            {
               "key": "brown",
               "doc_count": 3
            }
         ]
      }
   }
}
```
Ok, I see that now. My concern was that the test is checking the value of the others doc count in an indirect way (from a readability point of view) since we are checking sum of all the doc counts rather than directly checking the other doc count, but actually there probably isn't another way to do it.