channeldb+lnwallet: add ShutdownInfo with read and write methods

ShutdownInfo contains any info about a previous Shutdown message that we
have sent. This commit adds this type along with read and write methods
for it in the channel db. The existence of the ShutdownInfo on disk
represents the fact that we have previously sent the Shutdown message
and hence that we should resend it on re-establish.

lnwallet/chancloser/chancloser_test.go

@@ -154,6 +154,10 @@ func (m *mockChannel) MarkCoopBroadcasted(*wire.MsgTx, bool) error {
 	return nil
 }
 
+func (m *mockChannel) MarkShutdownSent(*channeldb.ShutdownInfo) error {
+	return nil
+}
+
 func (m *mockChannel) IsInitiator() bool {
 	return m.initiator
 }