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;