Require strict exchange of shutdown

Whenever one side sends `shutdown`, we restart a signing round from
scratch. To be compatible with future taproot channels, we require
the receiver to also send `shutdown` before moving on to exchanging
`closing_complete` and `closing_sig`. This will give nodes a message
to exchange fresh musig2 nonces before producing signatures.

On reconnection, we also restart a signing session from scratch and
discard pending partial signatures.

Author: t-bast

eclair-core/src/main/scala/fr/acinq/eclair/wire/internal/channel/version4/ChannelCodecs4.scala

@@ -780,7 +780,7 @@ private[channel] object ChannelCodecs4 {
     private val onRemoteShutdownCodec: Codec[OnRemoteShutdown] = discriminated[OnRemoteShutdown].by(uint8)
       .typecase(0x00, provide(OnRemoteShutdown.WaitForSigs))
       .typecase(0x01, feeratePerKw.as[OnRemoteShutdown.SignTransaction])
-    
+
     private val waitingForRemoteShutdownCodec: Codec[ClosingNegotiation.WaitingForRemoteShutdown] = (
       ("localShutdown" | lengthDelimited(shutdownCodec)) ::
         ("onRemoteShutdown" | onRemoteShutdownCodec)