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.
There is duplicate shard info headers from responses the originate from the same shard group, but that is expected. 

I can move it down to the else clause where ops are serialized.