`BlockStore.chain_head_pointers` gets confused when the same chain is in multiple stores

The method BlockStore.chain_head_pointers iterates over all shards slurping up information about chain head pointers without checking whether that data actually comes from the shard that is configured for any given chain. If a chain is present in more than one store (e.g., because it was moved to another shard) the information returned can be incorrect since old data might clobber new data.

The method needs to make sure that it only reads data from the correct shards. That's complicated a bit by the fact that we want to read all chain head pointers from a shard at once for performance, i.e. one query per shard, rather than one query per chain.

まず BlockStore.chain_head_pointers を見つけ、複数の shard にわたって chain head pointer をどのように収集しているかを読みます。各 chain の設定済み shard がどのように表現されているかを追跡し、その後、1 つの chain が複数の store に存在する場合の結果を検証します。完了の条件は、shard ごとに 1 回のクエリが維持され、古いデータが設定済み shard のデータを上書きできないことです。