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.
grrr...you are right @bleskes sorry for the confusion. It would be a breaking change for the java API. I don't like it when internal requests get exposed as they are to the outside world :)