Previously lua on_shutdown triggers were started sequentially, now
each of triggers starts in a separate fiber. Tarantool waits for 3.0
seconds to their completion by default. User has the option to change
this value using new implemented box.ctl.set_on_shutdown_timeout function.
If timeout has expired, tarantool immediately stops, without waiting for
other triggers completion.
Also moved ev_break from trigger to the on_shutdown_f function, after
calling all on_shutdown lua triggers, because now all triggers are
started asynchronously in fibers, and we should call ev_break only
after all triggers are finished.

Part of #5723

@TarantoolBot document
Title: Changed Lua on_shutdown triggers behaviour.
Previously lua on_shutdown triggers were started sequentially, now
each of triggers starts in a separate fiber. Tarantool waits for 3.0
seconds to their completion by default. User has the option to change
this value using new implemented box.ctl.set_on_shutdown_timeout function.
If timeout has expired, tarantool immediately stops, without waiting for
other triggers completion.