workers.queuecleanup: remove direct peewee usage

data/queue.py

@@ -274,3 +274,21 @@ class WorkQueue(object):
       except QueueItem.DoesNotExist:
         return
 
+
+def delete_expired(expiration_threshold, deletion_threshold, batch_size):
+  """
+  Deletes all queue items that are older than the provided expiration threshold in batches of the
+  provided size. If there are less items than the deletion threshold, this method does nothing.
+
+  Returns the number of items deleted.
+  """
+  to_delete = list(QueueItem
+                   .select()
+                   .where(QueueItem.processing_expires <= expiration_threshold)
+                   .limit(batch_size))
+
+  if len(to_delete) < deletion_threshold:
+    return 0
+
+  QueueItem.delete().where(QueueItem.id << to_delete).execute()
+  return len(to_delete)