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/solarconditions/http_solar_conditions_provider.py

@@ -25,11 +25,15 @@ class HTTPSolarConditionsProvider(SolarConditionsProvider):
 
     def start(self):
         logger.info(f"Set up query of {self.name} solar conditions API every {self._poll_interval!s} seconds.")
-        self._thread = Thread(target=self._run, name=f"HTTPSolarConditionsProvider-{self.name}")
+        self._thread = Thread(target=self._run, name=f"HTTPSolarConditionsProvider-{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} solar conditions worker thread did not exit on time and will be killed.")
 
     def _run(self):
         while True: