Add option for acknowledging failed tasks (globally and per-task)

If a task fails with an exception, it is retried until it
succeeds. This is contrary to what is said in the documentation: under
the "Architecture" section, heading "Broker" it says that even when a
task errors, it's still considered a successful delivery. Failed tasks
never get acknowledged however, thereby being retried after the
timeout period. See also issues #238 and #194.

This patch adds an option to acknowledge failures, thereby closing
issue #238. Issue #194 would require some more work. The default of
this option is set to `False`, thereby maintaining backwards
compatibility.

django_q/tasks.py

@@ -20,7 +20,7 @@ from django_q.queues import Queue
 def async(func, *args, **kwargs):
     """Queue a task for the cluster."""
     keywords = kwargs.copy()
-    opt_keys = ('hook', 'group', 'save', 'sync', 'cached', 'iter_count', 'iter_cached', 'chain', 'broker')
+    opt_keys = ('hook', 'group', 'save', 'sync', 'cached', 'ack_failure', 'iter_count', 'iter_cached', 'chain', 'broker')
     q_options = keywords.pop('q_options', {})
     # get an id
     tag = uuid()
@@ -42,6 +42,8 @@ def async(func, *args, **kwargs):
         task['cached'] = Conf.CACHED
     if 'sync' not in task and Conf.SYNC:
         task['sync'] = Conf.SYNC
+    if 'ack_failure' not in task and Conf.ACK_FAILURES:
+        task['ack_failure'] = Conf.ACK_FAILURES
     # finalize
     task['kwargs'] = keywords
     task['started'] = timezone.now()