box: implement on_shutdown triggers

Add on_shutdown triggers which are run by a preallocated fiber on
shutdown and make it possible to register them via box.ctl.on_shutdown()
Make use of the new triggers: now dedicate an on_shutdown trigger to
break event loop instead of doing it explicitly from signal handler.
The trigger is run last, so that all other on_shutdown triggers may
yield, sleep and so on.
Also make sure we can register lbox_triggers without push_event function
in case we don't need one.

Part of #1607