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 used `-1` to distinguish between noop operations and operations that actually should write on some shards, but maybe a noop should just use `0` as total... let me change that.