`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.

Inizia individuando BlockStore.chain_head_pointers e leggendo come raccoglie i puntatori alla testa delle chain tra gli shard. Segui il modo in cui è rappresentato lo shard configurato di ogni chain, quindi verifica il risultato quando una chain esiste in più store; il lavoro è completato quando rimane una query per shard e i dati obsoleti non possono sovrascrivere i dati dello shard configurato.