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.
I'll add it back :-) 

(my idea was that failures.length would be enough, but that wouldn't be consistent with the _shards header in search and percolate apis)