eth, p2p/msgrate: move peer QoS tracking to its own package and use it for snap (#22876)

This change extracts the peer QoS tracking logic from eth/downloader, moving
it into the new package p2p/msgrate. The job of msgrate.Tracker is determining
suitable timeout values and request sizes per peer.

The snap sync scheduler now uses msgrate.Tracker instead of the hard-coded 15s
timeout. This should make the sync work better on network links with high latency.