Rename total_score to nonce in sync message

[remagpie]

This PR introduces a change in the sync extension message interpretation.
The total_score in Status message is now interpreted as a `nonce.

When the snapshot sync lands, the total_score of a block doesn't mean the total accumulated score from the genesis block anymore.
To handle this, we need to avoid relying on the total_score for synchronization.

There were two parts to change:

1. Checking if the peer is leading or not

• This is now handled by checking the best_hash in the local blockchain.

2. Checking if we need to update the peer status or not

• This is now handled by nonce in the status message.

There are no changes in the message format itself, so I think it is a backward-compatible patch.

[majecty]

It's good to use nonce instead of the total score. total_score isn't saved in the state and need the whole history to calculate it. It is hard to manage and verify total_score.

This commit removes last_request and uses nonce instead. Sync extension is using last_requset to give a unique id of requests. The sync extension will work even though we use nonce instead. However, one variable used by two different purposes makes maintenance hard. IMO using two variables for each purpose will make code maintainable.

[remagpie]

It's good to use nonce instead of the total score. total_score isn't saved in the state and need the whole history to calculate it. It is hard to manage and verify total_score.

This commit removes last_request and uses nonce instead. Sync extension is using last_requset to give a unique id of requests. The sync extension will work even though we use nonce instead. However, one variable used by two different purposes makes maintenance hard. IMO using two variables for each purpose will make code maintainable.

Agreed. Let me separate those variables.

[majecty]

Shouldn't we update the self.nonce when a new block is imported?
self.nonce was updated correctly.