process_queue: For threaded workers, create them when they start.

Creating the QueueProcessingWorker objects when the ThreadedWorker is
created can lead to a race which caused confusing error messages:

1. A thread tries to call `self.worker = get_worker()`
2. This call raises an exception, which is caught by
   `log_and_exit_if_exception`
3. `log_and_exit_if_exception` sends our process a SIGUSR1, _but
    otherwise swallows the error_.
4. The thread's `.run()` is called, which tries to access
   `self.worker`, which was never set, and throws another exception.
5. The process handles the SIGUSR1, restarting.

Move the creation of the worker to when it is started, so the worker
object does not need to be stored, and possibly have a decoupled
failure.

zerver/management/commands/process_queue.py

@@ -119,15 +119,13 @@ class ThreadedWorker(threading.Thread):
         self.logger = logger
         self.queue_name = queue_name
 
-        with log_and_exit_if_exception(logger, queue_name, threaded=True):
-            self.worker = get_worker(queue_name, threaded=True)
-
     @override
     def run(self) -> None:
         with configure_scope() as scope, log_and_exit_if_exception(
             self.logger, self.queue_name, threaded=True
         ):
-            scope.set_tag("queue_worker", self.worker.queue_name)
-            self.worker.setup()
-            logging.debug("starting consuming %s", self.worker.queue_name)
-            self.worker.start()
+            scope.set_tag("queue_worker", self.queue_name)
+            worker = get_worker(self.queue_name, threaded=True)
+            worker.setup()
+            logging.debug("starting consuming %s", self.queue_name)
+            worker.start()