gracefully terminate web server

In previous revisions server was terminated by itself, thus no lock or
socket was removed. In new version, graceful termination of the queue
has been added as well as server now handles singals

src/ahriman/application/handlers/web.py

@@ -55,3 +55,7 @@ class Web(Handler):
 
         application = setup_service(architecture, configuration, spawner)
         run_server(application)
+
+        # terminate spawn process at the last
+        spawner.stop()
+        spawner.join()