multi: Set and check upfront shutdown address on close

This commit sets our close addresss to the address
specified by option upfront shutdown, if specified,
and disconnects from peers that fail to provide
their upfront shutdown address for coopertaive closes
of channels that were opened with the option set.

lnwallet/channel.go

@@ -4918,6 +4918,18 @@ func (lc *LightningChannel) ShortChanID() lnwire.ShortChannelID {
 	return lc.channelState.ShortChanID()
 }
 
+// LocalUpfrontShutdownScript returns the local upfront shutdown script for the
+// channel. If it was not set, an empty byte array is returned.
+func (lc *LightningChannel) LocalUpfrontShutdownScript() lnwire.DeliveryAddress {
+	return nil
+}
+
+// RemoteUpfrontShutdownScript returns the remote upfront shutdown script for the
+// channel. If it was not set, an empty byte array is returned.
+func (lc *LightningChannel) RemoteUpfrontShutdownScript() lnwire.DeliveryAddress {
+	return nil
+}
+
 // genHtlcScript generates the proper P2WSH public key scripts for the HTLC
 // output modified by two-bits denoting if this is an incoming HTLC, and if the
 // HTLC is being applied to their commitment transaction or ours.