Sketch of implementation:

1. Extend PageserverFeedback to include shard number & count
2. Update safekeeper structures that store a remote_consistent_lsn to have some type that stores an mapping of shard to remote consistent lsn, and can give a sensible min() of the remote_consistent_lsn if it has seen all the shards in the tenant.
3. Maps of shard->remote_consistent_lsn also need to handle shard splitting, i.e. one might initially have shard (0, count=1), and then get updates for shards (0, count=2) and (1, count=2) -- once we see a higher-count shard identity, we must throw away the lower-count ones.