Event callbacks can register or unregister new callbacks for the same event
while executing, and the previous triggering implementation allowed for event
callbacks to be inadvertently skipped.
The fix is to make a copy of the list of callbacks before executing any of
them. With this change, the resulting semantics are simple: any callbacks
registered before triggering are executed, and any new callbacks registered
are only visible at the next triggering of the event.
Note that this could potentially break existing callers who expected
newly-appended callbacks were immediately executed.
Fixes #9447. Originally based on a patch by @marksandler2.