Propose every nth block as finality candidate instead of all the blocks

[gaudenzkessler]

belongs to #1003

[haerdib]

To how I understood integritee-network/worker#1003 the following should be done in the parentchain:

• store the next_finalization_candidate_block_number or whatever you call it, it's a u64, so that's fine, somewhere as a state.
• Check if the block_number sent with this xt, matches the previously stored next_finalization_candidate_block_number. If it does, finalize that block hash.

This PR however adds a diff and checks if that diff plus the last finalized (?) block number match the current block number. If so, the that block is finalized. So the finalization of the block depends on the block_diff sent by the very same xt, not the one sent previously. Or did I misunderstand something?

We can take that offline if you'd like to, I think I finally understood integritee-network/worker#1003

[gaudenzkessler]

ok I can turn it around.

[clangenb]

Looks good in general. I have some minor remarks though. :)

[OverOrion]

LGTM, thank you, I especially liked that the function finalize_blocks_from_queue has been deleted. 🚀

[coax1d]

LGTM

[clangenb]

Nice, thanks for adding the asserts in the tests! Only one thing remains. :)

[clangenb]

Thanks for incorporating the feedback! I found one last thing. :)

[clangenb]

Can we please call the finalization candidate next_finalization_candidate? :)

[clangenb]

The naming of your test made me realize one more test that we need to do:

ensure!(
	next_finalization_candidate_block_number > finalization_candidate_block_number
	<Error<T>>::InvalidNextFinalizationCandidateBlockNumber
);