finalization: Skip tree route calculation if no forks present

[skunert]

Issue

Currently, syncing parachains from scratch can lead to a very long finalization time once they reach the tip of the chain. The problem is that we try to finalize everything from 0 to the tip, which can be thousands or even millions of blocks.

We finalize sequentially and try to compute displaced branches during finalization. So for every block on the way, we compute an expensive tree route.

Proposed Improvements

In this PR, I propose improvements that solve this situation:

• Skip tree route calculation if leaves().len() == 1: This should be enough for 90% of cases where there is only one leaf after sync.
• Optimize finalization for long distances: It can happen that the parachain has imported some leaf and then receives a relay chain notification with the finalized block. In that case, the previous optimization will not trigger. A second mechanism should ensure that we do not need to compute the full tree route. If the finalization distance is long, we check the lowest common ancestor of all the leaves. If it is above the to-be-finalized block, we know that there are no displaced leaves. This is fast because forks are short and close to the tip, so we can leverage the header cache.

Alternative Approach

• The problem was introduced in Change forks pruning algorithm. #3962. Reverting that PR is another possible strategy.
• We could store for every fork where it begins, however sounds a bit more involved to me.

fixes #4614

[burdges]

Around this, how do we hash headers?

It's often problematic if we do H(big_header) with parent in big_header. It's often faster if we do H(small_header, H(big_header)) with the parent hash in small_header.

[lexnv]

LGTM!