There's a case where a dropped funding_locked will result in the peer
moving onto channeld, while we stay in dualopend. As we haven't
received their funding_locked, we retransmit tx_sigs, which channeld
will need to handle.
With the patch the peer drops it on the floor; the peer will resend
funding_locked on reconnect, which will correctly advance us to
channeld and CHANNELD_NORMAL