Do not generate a channel-closed mon update for never-signed chans

Do not generate a channel-closed mon update for never-signed chans

The full_stack_target managed to find a bug where, if we receive
a funding_created message which has a channel_id identical to an
existing channel, we'll end up
 (a) having the monitor update for the new channel fail (due to
     duplicate outpoint),
 (b) creating a monitor update for the new channel as we
     force-close it,
 (c) panicing due to the force-close monitor update is applied to
     the original channel and is considered out-of-order.

Obviously we shouldn't be creating a force-close monitor update for
a channel which can never appear on chain, so we do that here and
add a test which previously failed and checks a few
duplicate-channel-id cases.