Changelog-Fixed: Plugins: pay now correctly estimates channel capacity
after payment failure, fixing some retry cases.
The `estimated_capacity` was properly substracted from the channel
hint, but adding the amount back did not work. The amount was added back
and then immediately substracted again. This caused the
`estimated_capacity` to ever decrease. This commit makes sure re-adding
the amount to the `estimated_capacity` works as expected.
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