eth, les: defer starting LES service until ETH initial sync is finished

eth/backend.go

@@ -104,6 +104,7 @@ type Config struct {
 
 type LesServer interface {
 	Start(srvr *p2p.Server)
+	Synced()
 	Stop()
 	Protocols() []p2p.Protocol
 }
@@ -145,6 +146,7 @@ type Ethereum struct {
 
 func (s *Ethereum) AddLesServer(ls LesServer) {
 	s.lesServer = ls
+	s.protocolManager.lesServer = ls
 }
 
 // New creates a new Ethereum object (including the