les: UDP pre-negotiation of available server capacity (#22183)

This PR implements the first one of the "lespay" UDP queries which
is already useful in itself: the capacity query. The server pool is making
use of this query by doing a cheap UDP query to determine whether it is
worth starting the more expensive TCP connection process.

p2p/nodestate/nodestate.go

@@ -599,6 +599,7 @@ func (ns *NodeStateMachine) updateEnode(n *enode.Node) (enode.ID, *nodeInfo) {
 	node := ns.nodes[id]
 	if node != nil && n.Seq() > node.node.Seq() {
 		node.node = n
+		node.dirty = true
 	}
 	return id, node
 }