container-sync doesn't work with sharded container

[Alistair Coles]

Container sync reads object rows directly from the source container DB. Once a root container is sharded, the 'fresh' root DB will not generally* receive object updates, so the container sync process will not find new object rows.

Container sync metadata (e.g. X-Container-Sync-To) is not copied to shard containers, so these will not be sync'd to the destination.

(* Some misplaced object updates will be written to the fresh DB.)