Log initial sync stats on completion

From https://github.com/erigontech/erigon/issues/16317#issuecomment-3196110618, I think the Downloader could log a summary of the initial sync for more quantifiable performance:

- How many incomplete files and bytes were finished (since the user may have resumed a previously incomplete node)
- How many bytes were hashed/skipped (sparse files optimization)
- Average webseed download rate
- Average p2p download/upload rates
- Chain name, and some kind of tag/representation for which preverified hash set was used as this is often omitted.

It's a small set of the most useful values to eyeball in CI and from users and easily compared. If it's a single log line it can be copy-pasted easily.