All the core notifications changed over to wrapping the notification
fields in an object with the name of the notification, but notifications
from plugins were missed.
Changelog-Added: Plugins: `channel_hint_update`, `pay_failure` and `pay_success` notifications now have objects of the same name containing the expected fields.
Changelog-Deprecated: Plugins: `channel_hint_update`, `pay_failure` and `pay_success` notification fields outside the same-named object.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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