[Keep] keepstore identifies underlying volumes to clients
The problem today: Keep clients figure if they write a block to a Keepstore, that created X-Keep-Replicas-Stored unique copies (default 1). This assumption is broken when multiple Keepstores share underlying storage (e.g., an object store).
Make it possible for clients to know how many copies of the block exist on disk.
- PUT responses should list which volume(s) the block was stored on, and how many copies live on each.
- Index responses should list which volume(s) each block lives on, and how many copies live on that volume.