We bias by channel linearly by capacity, scaled by median fee.
This means that we effectively double the fee if we would use the
entire capacity, and only increase it by 50% if we would only use
1/2 the capacity.
This should drive us towards larger channels.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: Plugins: `pay` now biases towards larger channels, improving success probability.
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