p2p/discover: improve table addition code (#18974)

This change clears up confusion around the two ways in which nodes
can be added to the table.

When a neighbors packet is received as a reply to findnode, the nodes
contained in the reply are added as 'seen' entries if sufficient space
is available.

When a ping is received and the endpoint verification has taken place,
the remote node is added as a 'verified' entry or moved to the front of
the bucket if present. This also updates the node's IP address and port
if they have changed.

p2p/discover/table_util_test.go

@@ -86,17 +86,8 @@ func fillBucket(tab *Table, n *node) (last *node) {
 // fillTable adds nodes the table to the end of their corresponding bucket
 // if the bucket is not full. The caller must not hold tab.mutex.
 func fillTable(tab *Table, nodes []*node) {
-	tab.mutex.Lock()
-	defer tab.mutex.Unlock()
-
 	for _, n := range nodes {
-		if n.ID() == tab.self().ID() {
-			continue // don't add self
-		}
-		b := tab.bucket(n.ID())
-		if len(b.entries) < bucketSize {
-			tab.bumpOrAdd(b, n)
-		}
+		tab.addSeenNode(n)
 	}
 }
 
@@ -154,15 +145,6 @@ func hasDuplicates(slice []*node) bool {
 	return false
 }
 
-func contains(ns []*node, id enode.ID) bool {
-	for _, n := range ns {
-		if n.ID() == id {
-			return true
-		}
-	}
-	return false
-}
-
 func sortedByDistanceTo(distbase enode.ID, slice []*node) bool {
 	var last enode.ID
 	for i, e := range slice {