Use batch add for Tumble task queue tasks.

tumble/fire_tasks.go

 // Copyright 2015 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package tumble
 
 import (
         "fmt"
         "math"
         "time"
@@ skipping 60 matching lines @@
                         Path: processURL(eta, shard.shard),
 
                         ETA: eta.Unix(),
 
                         // TODO(riannucci): Tune RetryOptions?
                 }
                 tasks = append(tasks, tsk)
                 logging.Infof(c, "added task %q %s %s", tsk.Name, tsk.Path, tsk.ETA)
         }
 
-        if err := errors.Filter(tq.Add(ds.WithoutTransaction(c), baseName, tasks...), tq.ErrTaskAlreadyAdded); err != nil {
+        b := tq.Batcher{}
+        if err := errors.Filter(b.Add(ds.WithoutTransaction(c), baseName, tasks...), tq.ErrTaskAlreadyAdded); err != nil {
                 logging.Warningf(c, "attempted to fire tasks %v, but failed: %s", shards, err)
                 return false
         }
         return true
 }