Finish closing channel after async closing signed

Finish closing channel after async closing signed

In addressing a followup to test reconnection during closing negotation
with async signing, we change things to only return a `ShutdownResult`
when we actually finish shutting down the channel, i.e. we have the
signature ready to send the final closing signed. This slightly
simplifies the logic where we would shutdown our channel
prematurely before we got the final signature. This also means
that we don't push multiple `ChannelClosed` events if we receive closing
signed, reconnect, and receive closing signed again.