stop birpc listeners on engine shutdown

make goroutine wait on shdChan and call StopBiRPC to close the
listener(s). Connection goroutines still leak (they need to be
fixed on birpc library side).

services/engine.go

@@ -264,6 +264,10 @@ func startBiRPC(smg *SessionService, tS *ThresholdService, server *cores.Server,
 			}
 		},
 	}
+	go func() {
+		<-shdChan.Done()
+		server.StopBiRPC()
+	}()
 	if err := server.ServeBiRPC(cfg.ListenCfg().BiJSONListen, cfg.ListenCfg().BiGobListen, onConns, onDiss); err != nil {
 		utils.Logger.Err(fmt.Sprintf("<%s> serve BiRPC error: %s!", utils.SessionS, err))
 		shdChan.CloseOnce()