avoid data race read volume.IsEmpty (#4574)

* avoid data race read volume.IsEmpty

-   avoid phantom read isEmpty for onlyEmpty
-   use `v.DataBackend.GetStat()` in v.dataFileAccessLock scope

* add Destroy(onlyEmpty: true) test

* add Destroy(onlyEmpty: false) test

* remove unused `IsEmpty()`

* change literal `8` to `SuperBlockSize`

weed/shell/command_volume_delete_empty.go

@@ -5,6 +5,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/pb"
 	"github.com/seaweedfs/seaweedfs/weed/pb/master_pb"
 	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
+	"github.com/seaweedfs/seaweedfs/weed/storage/super_block"
 	"io"
 	"log"
 	"time"
@@ -57,7 +58,7 @@ func (c *commandVolumeDeleteEmpty) Do(args []string, commandEnv *CommandEnv, wri
 	eachDataNode(topologyInfo, func(dc string, rack RackId, dn *master_pb.DataNodeInfo) {
 		for _, diskInfo := range dn.DiskInfos {
 			for _, v := range diskInfo.VolumeInfos {
-				if v.Size <= 8 && v.ModifiedAtSecond > 0 && v.ModifiedAtSecond+quietSeconds < nowUnixSeconds {
+				if v.Size <= super_block.SuperBlockSize && v.ModifiedAtSecond > 0 && v.ModifiedAtSecond+quietSeconds < nowUnixSeconds {
 					if *applyBalancing {
 						log.Printf("deleting empty volume %d from %s", v.Id, dn.Id)
 						if deleteErr := deleteVolume(commandEnv.option.GrpcDialOption, needle.VolumeId(v.Id),