sstables: select just the shard we want when writing sharding metadata
sstables: select just the shard we want when writing sharding metadata

On a system with many shards, this saves many useless iterations where
we just skip the unwanted shard.

duarten/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla