les, les/lespay: implement new server pool (#20758)

This PR reimplements the light client server pool. It is also a first step
to move certain logic into a new lespay package. This package will contain
the implementation of the lespay token sale functions, the token buying and
selling logic and other components related to peer selection/prioritization
and service quality evaluation. Over the long term this package will be
reusable for incentivizing future protocols.

Since the LES peer logic is now based on enode.Iterator, it can now use
DNS-based fallback discovery to find servers.

This document describes the function of the new components:
https://gist.github.com/zsfelfoldi/3c7ace895234b7b345ab4f71dab102d4

les/lespay/client/valuetracker.go

@@ -213,6 +213,15 @@ func (vt *ValueTracker) StatsExpirer() *utils.Expirer {
 	return &vt.statsExpirer
 }
 
+// StatsExpirer returns the current expiration factor so that other values can be expired
+// with the same rate as the service value statistics.
+func (vt *ValueTracker) StatsExpFactor() utils.ExpirationFactor {
+	vt.statsExpLock.RLock()
+	defer vt.statsExpLock.RUnlock()
+
+	return vt.statsExpFactor
+}
+
 // loadFromDb loads the value tracker's state from the database and converts saved
 // request basket index mapping if it does not match the specified index to name mapping.
 func (vt *ValueTracker) loadFromDb(mapping []string) error {
@@ -500,16 +509,3 @@ func (vt *ValueTracker) RequestStats() []RequestStatsItem {
 	}
 	return res
 }
-
-// TotalServiceValue returns the total service value provided by the given node (as
-// a function of the weights which are calculated from the request timeout value).
-func (vt *ValueTracker) TotalServiceValue(nv *NodeValueTracker, weights ResponseTimeWeights) float64 {
-	vt.statsExpLock.RLock()
-	expFactor := vt.statsExpFactor
-	vt.statsExpLock.RUnlock()
-
-	nv.lock.Lock()
-	defer nv.lock.Unlock()
-
-	return nv.rtStats.Value(weights, expFactor)
-}