tui/remote_ui: Fix some regressions

- Explicitly set the SignalWatcher event queue. Without this, the watcher will
  publish events to the fast queue, resulting in resize bugs for certain
  terminals(#2322).
- Set `async = false` to the `remote_ui_attach` handler(It was a deferred
  before, this is the new equivalent)

src/nvim/msgpack_rpc/remote_ui.c

@@ -28,7 +28,7 @@ void remote_ui_init(void)
   connected_uis = pmap_new(uint64_t)();
   // Add handler for "attach_ui"
   String method = cstr_as_string("ui_attach");
-  MsgpackRpcRequestHandler handler = {.fn = remote_ui_attach, .async = true};
+  MsgpackRpcRequestHandler handler = {.fn = remote_ui_attach, .async = false};
   msgpack_rpc_add_method_handler(method, handler);
   method = cstr_as_string("ui_detach");
   handler.fn = remote_ui_detach;

src/nvim/tui/tui.c

@@ -140,6 +140,7 @@ UI *tui_start(void)
 
   // listen for SIGWINCH
   signal_watcher_init(&loop, &data->winch_handle, ui);
+  data->winch_handle.events = queue_new_child(loop.events);
   signal_watcher_start(&data->winch_handle, sigwinch_cb, SIGWINCH);
 
   ui->stop = tui_stop;
@@ -179,6 +180,7 @@ static void tui_stop(UI *ui)
   // Destroy common stuff
   kv_destroy(data->invalid_regions);
   signal_watcher_stop(&data->winch_handle);
+  queue_free(data->winch_handle.events);
   signal_watcher_close(&data->winch_handle, NULL);
   // Destroy input stuff
   term_input_destroy(data->input);