Integrate scheduler into dispatcher main loop (#14067)

Dispatcher refactoring to get pg_notify publish payload
  as separate method

Refactor periodic module under dispatcher entirely
  Use real numbers for schedule reference time
  Run based on due_to_run method

Review comments about naming and code comments

awx/main/dispatch/control.py

@@ -40,6 +40,9 @@ class Control(object):
     def cancel(self, task_ids, *args, **kwargs):
         return self.control_with_reply('cancel', *args, extra_data={'task_ids': task_ids}, **kwargs)
 
+    def schedule(self, *args, **kwargs):
+        return self.control_with_reply('schedule', *args, **kwargs)
+
     @classmethod
     def generate_reply_queue_name(cls):
         return f"reply_to_{str(uuid.uuid4()).replace('-','_')}"
@@ -52,14 +55,14 @@ class Control(object):
         if not connection.get_autocommit():
             raise RuntimeError('Control-with-reply messages can only be done in autocommit mode')
 
-        with pg_bus_conn() as conn:
+        with pg_bus_conn(select_timeout=timeout) as conn:
             conn.listen(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):
+            for reply in conn.events(yield_timeouts=True):
                 if reply is None:
                     logger.error(f'{self.service} did not reply within {timeout}s')
                     raise RuntimeError(f"{self.service} did not reply within {timeout}s")