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.
good point, we (core) run on > 2 nodes all the time, but clients tests run on a single node cluster most of the time, thus the rest tests need to run against a single node. the one replica and wait for green would indeed make clients build fail.