refactoring to get master function, instead of passing master values directly

this will enable retrying later

weed/server/volume_server_handlers_read.go

@@ -63,7 +63,7 @@ func (vs *VolumeServer) GetOrHeadHandler(w http.ResponseWriter, r *http.Request)
 			w.WriteHeader(http.StatusNotFound)
 			return
 		}
-		lookupResult, err := operation.Lookup(vs.GetMaster(), volumeId.String())
+		lookupResult, err := operation.Lookup(vs.GetMaster, volumeId.String())
 		glog.V(2).Infoln("volume", volumeId, "found on", lookupResult, "error", err)
 		if err == nil && len(lookupResult.Locations) > 0 {
 			u, _ := url.Parse(util.NormalizeUrl(lookupResult.Locations[0].PublicUrl))