If we add a new hook, not at the end, while hooks are getting called,
then iteration could be messed up (e.g. calling a plugin twice, or
skipping one).
The simplest thing is to defer updates until nobody is calling the
hook. In theory this could livelock, in practice it won't.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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