This PR contains two changes:
Firstly, the finalized header will be resolved from local chain if it's
not recently announced via the `engine_newPayload`.
What's more importantly is, in the downloader, originally there are two
code paths to push forward the pivot point block, one in the beacon
header fetcher (`fetchHeaders`), and another one is in the snap content
processer (`processSnapSyncContent`).
Usually if there are new blocks and local pivot block becomes stale, it
will firstly be detected by the `fetchHeaders`. `processSnapSyncContent`
is fully driven by the beacon headers and will only detect the stale pivot
block after synchronizing the corresponding chain segment. I think the
detection here is redundant and useless.
27c4ca9df0