Events need to be cancelled when an fd is released,
to avoid possible memory leaks or use after free.
When the event is cancelled, its callback is called.
Currently this is sufficient since events are used for
resource management and we have no option but to
release the lock or memory. If future uses need to
distinguish between the callback firing and
a cancel, they can look at the timestamp passed to
the callback, which will be before the timestamp they
expected. Otherwise a separate cancel callback can
be added.
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