fix race detector found problems

weed/storage/needle_map_memory.go

@@ -47,9 +47,7 @@ func LoadBtreeNeedleMap(file *os.File) (*NeedleMap, error) {
 
 func doLoading(file *os.File, nm *NeedleMap) (*NeedleMap, error) {
 	e := WalkIndexFile(file, func(key NeedleId, offset Offset, size uint32) error {
-		if key > nm.MaximumFileKey {
-			nm.MaximumFileKey = key
-		}
+		nm.MaybeSetMaxFileKey(key)
 		if !offset.IsZero() && size != TombstoneFileSize {
 			nm.FileCounter++
 			nm.FileByteCounter = nm.FileByteCounter + uint64(size)
@@ -67,7 +65,7 @@ func doLoading(file *os.File, nm *NeedleMap) (*NeedleMap, error) {
 		}
 		return nil
 	})
-	glog.V(1).Infof("max file key: %d for file: %s", nm.MaximumFileKey, file.Name())
+	glog.V(1).Infof("max file key: %d for file: %s", nm.MaxFileKey(), file.Name())
 	return nm, e
 }