dl: mutex and stats follow-up

follow-up after: https://github.com/erigontech/erigon/pull/16895/files

- unclear which fields it covering - group such fields to 1 structure with mutex
- add explicit `Clone` method to `Stats` class. Because it has pointers and we do copy by `prevStats := dl.stats`
- avoid atomics use in `Stats` class or make all fields atomic
- re-check perf of `Downloader.newStats` method