Initial header sync (request headers from fast peers or avoid slow peers)

The initial sync feels slow. We request headers in chunks of 512 from random connected peer. Sometimes we hit a slow peer and the latency introduced in even a few requests from a slow peer makes the header sync feel slow.

It might be nice to track peer speeds and make a better decision in terms of prioritizing faster peers as we proceed with the header sync.

We already track download rates at some level of granularity (not entirely sure what we do today) as we display this on the TUI - so we may already collect enough information to make this decision around which peers are best to request headers from.