It handles all the cases of retransmission, and in the normal case
retransmits shutdown and immediately returns for us to run closingd.
This is actually far simpler and reduces code duplication.
[ Includes fixup to stop warn_unused_result from Christian ]
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: Protocol: We could get stuck on signature exchange if we needed to retransmit the final revoke_and_ack.
v2 channel open uses a different method to derive the channel_id, so now
we save it to the database so that we dont have to remember how to
derive it for each.
includes a migration for existing channels
