[app_server] deadlock on workspace switching

[diver]

This is hard to reproduce i fear, but i'll try to explain. This is kinda stress test and takes about 3-5 min. After Haiku bootup type in terminal - screensaver. You need to switch workspaces (alt+f1,alt+f2) like crazy and in the same time you need to switch betwen terminal and screensaver from in the team menu of the deskbar. You couls see various things, like some menus opening in the upper left corner, white spaces, etc. After several minutes app_server will just freeze.