Replace state.need_restart with state.server_command + replace poll loop with signal

modules/ui.py

@@ -1609,12 +1609,8 @@ def create_ui():
             outputs=[]
         )
 
-        def request_restart():
-            shared.state.interrupt()
-            shared.state.need_restart = True
-
         restart_gradio.click(
-            fn=request_restart,
+            fn=shared.state.request_restart,
             _js='restart_reload',
             inputs=[],
             outputs=[],