eth: resolve finalized from disk if it's not recently announced

[rjl493456442]

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.

[rjl493456442]

We should test the snap sync before merging it, open it for gathering feedbacks