We were basing the 2016 block timeout on the blockchain height that we had
processed at the time completed the funding, which could lag considerably
behind the network-wide blockheight. For example this could happen if we
started with `--rescan` from a low height, taking some time to catch up.
This means that we could end up forgetting channels even before reaching the
network blockheight. This patch instead uses the headercount reported by the
backend plugin if we aren't caught up yet. While the chances of this happening
are still there, the window it might happen are much reduced, since headers
can be synced in a couple of minutes.
Reported-by: Riccardo Masutti
Changelog-Changed: Funding timeout is now based on the header count reported by the bitcoin backend instead of our current blockheight which might be lower.
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