This was requested by Michael of Boltz; it's mainly useful if you plan to
try failed payments on a *different* node. In that case, there's a
theoretical possibility that slow parts of this payment could combine with
that from a different node and overpay.
We don't allow this from the same node, already.
Changelog-Added: xpay: `xpay-slow-mode` makes xpay wait for all parts of a payment to complete before returning success or failure.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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