Use _stop_event consistently across all threads as the way to signal that it should stop. Add thread joins with timeouts to allow the program to exit cleanly

providers/alert/http_alert_provider.py

@@ -27,11 +27,15 @@ class HTTPAlertProvider(AlertProvider):
         # Fire off the polling thread. It will poll immediately on startup, then sleep for poll_interval between
         # subsequent polls, so start() returns immediately and the application can continue starting.
         logger.info(f"Set up query of {self.name} alert API every {self._poll_interval!s} seconds.")
-        self._thread = Thread(target=self._run, name=f"HTTPAlertProvider-{self.name}")
+        self._thread = Thread(target=self._run, name=f"HTTPAlertProvider-{self.name}", daemon=True)
         self._thread.start()
 
     def stop(self):
         self._stop_event.set()
+        if self._thread:
+            self._thread.join(timeout=35)
+            if self._thread.is_alive():
+                logger.warning(f"{self.name} alert worker thread did not exit on time and will be killed.")
 
     def _run(self):
         while True: