Added `_shards` header to all write responses.

The header indicates to how many shard copies (primary and replicas shards) a write was supposed to go to, to how many shard copies to write succeeded and potentially captures shard failures if writing into a replica shard fails.

For async writes it also includes the number of shards a write is still pending.
same here - can we move the `_shards` to be the last of the `_` fields? Now it's mixed:

```
 {
         "index": {
            "_index": "index",
            "_type": "type",
            "_id": "3",
            "_version": 1,
            "status": 201,
            "_shards": {
               "total": 2,
               "successful": 1,
               "failed": 0
            }
         }
      },
```