Refactor canceling to work through messaging and signals, not database

If canceled attempted before, still allow attempting another cancel
in this case, attempt to send the sigterm signal again.
Keep clicking, you might help!

Replace other cancel_callbacks with sigterm watcher
  adapt special inventory mechanism for this too

Get rid of the cancel_watcher method with exception in main thread

Handle academic case of sigterm race condition

Process cancelation as control signal

Fully connect cancel method and run_dispatcher to control

Never transition workflows directly to canceled, add logs

awx/main/dispatch/control.py

@@ -37,18 +37,24 @@ class Control(object):
     def running(self, *args, **kwargs):
         return self.control_with_reply('running', *args, **kwargs)
 
+    def cancel(self, task_ids, *args, **kwargs):
+        return self.control_with_reply('cancel', *args, extra_data={'task_ids': task_ids}, **kwargs)
+
     @classmethod
     def generate_reply_queue_name(cls):
         return f"reply_to_{str(uuid.uuid4()).replace('-','_')}"
 
-    def control_with_reply(self, command, timeout=5):
+    def control_with_reply(self, command, timeout=5, extra_data=None):
         logger.warning('checking {} {} for {}'.format(self.service, command, self.queuename))
         reply_queue = Control.generate_reply_queue_name()
         self.result = None
 
         with pg_bus_conn(new_connection=True) as conn:
             conn.listen(reply_queue)
-            conn.notify(self.queuename, json.dumps({'control': command, 'reply_to': reply_queue}))
+            send_data = {'control': command, 'reply_to': reply_queue}
+            if extra_data:
+                send_data.update(extra_data)
+            conn.notify(self.queuename, json.dumps(send_data))
 
             for reply in conn.events(select_timeout=timeout, yield_timeouts=True):
                 if reply is None: