Initialize the worker boot sequence and start up all the
        worker threads.

def start(self):
        """Initialize the worker boot sequence and start up all the
        worker threads.
        """
        self.broker.emit_before("worker_boot", self)

        worker_middleware = _WorkerMiddleware(self)
        self.broker.add_middleware(worker_middleware)
        for _ in range(self.worker_threads):
            self._add_worker()

        self.broker.emit_after("worker_boot", self)