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.

eth/downloader/statesync.go

@@ -433,8 +433,8 @@ func (s *stateSync) assignTasks() {
 	peers, _ := s.d.peers.NodeDataIdlePeers()
 	for _, p := range peers {
 		// Assign a batch of fetches proportional to the estimated latency/bandwidth
-		cap := p.NodeDataCapacity(s.d.requestRTT())
-		req := &stateReq{peer: p, timeout: s.d.requestTTL()}
+		cap := p.NodeDataCapacity(s.d.peers.rates.TargetRoundTrip())
+		req := &stateReq{peer: p, timeout: s.d.peers.rates.TargetTimeout()}
 
 		nodes, _, codes := s.fillTasks(cap, req)