add "weed filer.cat" to read files directly from volume servers

weed/wdclient/vid_map.go

@@ -15,6 +15,12 @@ const (
 	maxCursorIndex = 4096
 )
 
+type HasLookupFileIdFunction interface {
+	GetLookupFileIdFunction() LookupFileIdFunctionType
+}
+
+type LookupFileIdFunctionType func(fileId string) (targetUrls []string, err error)
+
 type Location struct {
 	Url        string `json:"url,omitempty"`
 	PublicUrl  string `json:"publicUrl,omitempty"`
@@ -67,6 +73,10 @@ func (vc *vidMap) LookupVolumeServerUrl(vid string) (serverUrls []string, err er
 	return
 }
 
+func (vc *vidMap) GetLookupFileIdFunction() LookupFileIdFunctionType {
+	return vc.LookupFileId
+}
+
 func (vc *vidMap) LookupFileId(fileId string) (fullUrls []string, err error) {
 	parts := strings.Split(fileId, ",")
 	if len(parts) != 2 {