Join process on termination

Author: colin-nolan

postgresql_watcher/watcher.py

@@ -134,6 +134,7 @@ def _cleanup_connections_and_processes(self) -> None:
             self.child_conn = None
         if self.subscription_process is not None:
             self.subscription_process.terminate()
+            self.subscription_process.join()
             self.subscription_process = None
 
     def set_update_callback(self, update_handler: Optional[Callable[[None], None]]):