Improve encapsulation: Add shallowClone() method to vidMap

Added a shallowClone() method to vidMap to improve encapsulation and prevent
MasterClient from directly accessing vidMap's internal fields.

Changes:
1. Added vidMap.shallowClone() in vid_map.go
   - Encapsulates the shallow copy logic within vidMap
   - Makes vidMap responsible for its own state representation
   - Documented that caller is responsible for thread safety

2. Simplified resetVidMap() in masterclient.go
   - Uses tail := mc.vidMap.shallowClone() instead of manual field access
   - Cleaner, more maintainable code
   - Better adherence to encapsulation principles

Benefits:
- Improved code organization and maintainability
- vidMap internals are now properly encapsulated
- Easier to modify vidMap structure in the future
- More self-documenting code

Verified with: go test -race ./weed/wdclient/... (passes)

weed/wdclient/masterclient.go

@@ -592,14 +592,10 @@ func (mc *MasterClient) resetVidMap() {
 	mc.vidMapLock.Lock()
 	defer mc.vidMapLock.Unlock()
 
-	tail := &vidMap{
-		vid2Locations:   mc.vidMap.vid2Locations,
-		ecVid2Locations: mc.vidMap.ecVid2Locations,
-		DataCenter:      mc.vidMap.DataCenter,
-		cache:           mc.vidMap.cache,
-	}
+	// Create a shallow clone to preserve in the cache chain
+	tail := mc.vidMap.shallowClone()
 
-	nvm := newVidMap(mc.vidMap.DataCenter)
+	nvm := newVidMap(tail.DataCenter)
 	nvm.cache = tail
 	mc.vidMap = nvm
 

weed/wdclient/vid_map.go

@@ -53,6 +53,17 @@ func newVidMap(dataCenter string) *vidMap {
 	}
 }
 
+// shallowClone creates a shallow copy of the vidMap for use in cache chaining.
+// The caller is responsible for ensuring thread safety.
+func (vc *vidMap) shallowClone() *vidMap {
+	return &vidMap{
+		vid2Locations:   vc.vid2Locations,
+		ecVid2Locations: vc.ecVid2Locations,
+		DataCenter:      vc.DataCenter,
+		cache:           vc.cache,
+	}
+}
+
 func (vc *vidMap) getLocationIndex(length int) (int, error) {
 	if length <= 0 {
 		return 0, fmt.Errorf("invalid length: %d", length)