Generate callback for a queue

		:param signal_name: name of a signal that callback is generated for
		:type signal_name: str

		:rtype: callable

def __watchers_callbacks_exec(self, signal_name):
		""" Generate callback for a queue

		:param signal_name: name of a signal that callback is generated for
		:type signal_name: str

		:rtype: callable
		"""
		def callback_fn():
			for watcher in self.__watchers_callbacks[signal_name]:
				if watcher is not None:
					watcher.notify()
		return callback_fn