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.
Minor - can we move the _shards above CREATED? will look better. now looks like this:

```
{
   "_index": "index",
   "_type": "type",
   "_id": "1",
   "_version": 1,
   "created": true,
   "_shards": {
      "total": 2,
      "successful": 1,
      "failed": 0
   }
}
```