IF: Use finalizer diffs in instant_finality_extension

[heifner]

Instead of placing the entire finalizer set in the instant_finality_extension, only put a diff of the active finalizer policy in the extension.

[ericpassmore]

Note:start
group: IF
category: PERFORMANCE
summary: Instead of placing the entire finalizer set in the instant_finality_extension, only put a diff of the active finalizer policy in the extension.
Note:end

[greg7mdp]

I think ordered_diff should be templatized by size_type, because for policy diffs uint16_t is probably plenty enough to store in the remove_indexes and insert_indexes vectors.

[linh2931]

I started to wonder if it is worthy the trouble (time to diff and apply_diff, and more code) to pack diff only in the extension. Another side effect is one cannot just look at the extension to see the finalizers; previous extensions need to be checked.

[greg7mdp]

should be static_assert(config::max_finalizers <= std::numeric_limits<uint16_t>::max() + 1)

[heifner]

Yes, already had that in my changes for the next PR. Note you can't use max() + 1 because that overflows. So max_finalizers - 1.

[greg7mdp]

Good check!

No big deal, but I think this is a more accurate name.

Suggested change

	auto verify_block_finality_generation = [](const signed_block_ptr& block, uint32_t gen, const bls_public_key& key) {
	auto verify_block_finality_policy_diff = [](const signed_block_ptr& block, uint32_t gen, const bls_public_key& key) {