core: disable fibers creation after shutdown started

On Tarantool shutdown we cancel client fibers and then wait their
finishing (see the commit bf620650 ("box: finish client fibers on
shutdown"). If we allow to spawn new fibers after shutdown is started
then Tarantool shutdown can easily be hanged if spawned fiber does not
finish in time. This can be the case if client fiber ignores cancel
flag at the time of creating new fiber. I guess fiber does not need
creating another fiber to handle cancelling thus it is better to disable
it.

Part of #8423

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