We picked one node and iterated. This means we would only sent to 1,
not 2 nodes if we picked the last (common if there were only a few
peers). But it also means we would generally send to the same pair of
nodes, which isn't great for redundancy.
Rework to be more random.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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