finalization: Skip tree route calculation if no forks present (#4721)

## 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 #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