onchaind: replay using real block data, not db

[rustyrussell]

We have a report of onchaind not sweeping properly. It seems to be missing an output during replay, which makes sense since Greenlight restarts nodes (a lot!).

See https://github.com/Blockstream/greenlight/issues/433

I've also got a similar issue on my node, but I assumed that was me corrupting the database by editing it long ago. I will, however, test this fix on my node too...