refactor: drop canister_id from CanisterSnapshot struct

[mraszyk]

This PR drops the canister_id field from struct CanisterSnapshot. The motivation is two fold:

• the canister ID can be derived from the snapshot ID;
• canister snapshots are now stored in the corresponding CanisterState and thus the canister ID is unambiguous anyway.

This change only concerns the in-memory struct CanisterSnapshot. The on-disk representation is unchanged:

• such a change would be breaking;
• the on-disk representation represents the snapshot ID as a pair of the canister ID and the snapshot sequence number within the single canister and thus the on-disk representation is already minimal.