Thread the signed tx through so close's JSON return contains that,
rather than the unsigned channel->last_tx.
We have to split the "get cmd_id" from "resolve the close commands" though;
and of course, as before, we don't actually print the txids of multiple
transactions even though we may have multi in flight due to splice!
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: JSON-RPC: `close` returns a `tx` field with witness data populated (i.e. signed).
Fixes: #6440
e981a4f14a