Makes the callback variable "removed" atomic, and on script unload,
first sets all callbacks to removed before actually unloading the script
out of a safety precaution. (See note at the bottom for further details)
This minimizes the possibility of a race condition where the script
callback could be called while those callbacks were being removed.
Big note for this change, this change should eventually be replaced with
a reference counting ownership method where script callbacks can hold a
reference and share ownership of the script if it's still alive while
the script callback is being called. That way the script callbacks can
safely execute. May require a fair amount of reworking of the script
object.
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