add some String() to make codes easier to read and debug

go/topology/volume_location_list.go

@@ -1,5 +1,9 @@
 package topology
 
+import (
+	"fmt"
+)
+
 type VolumeLocationList struct {
 	list []*DataNode
 }
@@ -8,6 +12,10 @@ func NewVolumeLocationList() *VolumeLocationList {
 	return &VolumeLocationList{}
 }
 
+func (dnll *VolumeLocationList) String() string {
+	return fmt.Sprintf("%v", dnll.list)
+}
+
 func (dnll *VolumeLocationList) Head() *DataNode {
 	return dnll.list[0]
 }