UserManager::isRegisteredUser(JID) can block for a minute

As discussed in the thread, what seems to happen is

i) The user isn't a local user, and is not a cached remote user, so

ii) The UserManager sends a disco#info request to the remote server

iii) UserManager blocks, waiting for a response

iv) The disco#info result comes back in, but isn't processed by UserManager because it's blocked

v) 60 seconds after sending the request, UserManager times out and continues processing.