volume: change all writes to fsync during graceful stopping

fix https://github.com/chrislusf/seaweedfs/issues/2193

weed/command/volume.go

@@ -259,6 +259,7 @@ func (v VolumeServerOptions) startVolumeServer(volumeFolders, maxVolumeCounts, v
 
 		// Stop heartbeats
 		if !volumeServer.StopHeartbeat() {
+			volumeServer.SetStopping()
 			glog.V(0).Infof("stop send heartbeat and wait %d seconds until shutdown ...", *v.preStopSeconds)
 			time.Sleep(time.Duration(*v.preStopSeconds) * time.Second)
 		}