polygon/sync: Clean shutdown (#10017)

Author: shohamc1

eth/backend.go

@@ -1407,6 +1407,7 @@ func (s *Ethereum) Start() error {
 		s.waitForStageLoopStop = nil // TODO: Ethereum.Stop should wait for execution_server shutdown
 		go s.eth1ExecutionServer.Start(s.sentryCtx)
 	} else if s.config.PolygonSync {
+		s.waitForStageLoopStop = nil // Shutdown is handled by context
 		go func() {
 			ctx := s.sentryCtx
 			err := s.polygonSyncService.Run(ctx)