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'd consider adding a constructor with `shardInfo` and changing the subclasses constructor to accept it there, just to enforce that this info is needed so we don't forget it anywhere. Thoughts?