add sleep between upload retries

weed/operation/upload_content.go

@@ -100,6 +100,7 @@ func retriedUploadData(uploadUrl string, filename string, cipher bool, data []by
 		} else {
 			glog.Warningf("uploading to %s: %v", uploadUrl, err)
 		}
+		time.Sleep(time.Millisecond * time.Duration(237 * (i+1)))
 	}
 	return
 }