state sync: refactor structures identifying State Parts

[saketh-are]

In state sync a state part is generally identified by:

• A shard id
• A sync hash
• A part id

In various places we use types such as tuple (ShardId, CryptoHash, u64) without named members, and the order in which the fields are presented is inconsistent between places. We should improve the consistency and consider a proper struct with named members for better maintainability.