This will initially be for listinvoices, but can be expanded to other
list commands.
It's documented, but it makes promises which currently don't exist:
* listinvoice does not support `index` or `start` yet.
* It doesn't actually fire when invoices change yet.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `wait`: new generic command to wait for events.
