timers: don't mutate unref list while iterating it

Commit 934bfe23a16556d05bfb1844ef4d53e8c9887c3d had introduced a
regression where node would crash trying to access a null unref timer if
a given unref timer's callback would remove other unref timers set to
fire in the future.

More generally, it makes the unrefTimeout function more solid by not
mutating the unrefList while traversing it.

Fixes #8897.