Add DeleteEntitiesIter for viur-transactionmarker

Author: ArneGudermann

src/viur/core/tasks.py

@@ -869,3 +869,14 @@ def handleEntry(cls, entry, customData):
             entry.delete()
         else:
             db.Delete(entry.key)
+
+
+@PeriodicTask(interval=datetime.timedelta(hours=4))
+def start_clear_transaction_marker():
+    """
+        Removes old (expired) Transaction marker
+        https://cloud.google.com/datastore/docs/concepts/transactions?hl=en#using_transactions
+    """
+    query = db.Query("viur-transactionmarker").filter("creationdate <",
+                                                      datetime.datetime.now() - datetime.timedelta(seconds=270))
+    DeleteEntitiesIter.startIterOnQuery(query)