Correct Channel outbound HTLC serialization

Channel serialization should happen "as if
remove_uncommitted_htlcs_and_mark_paused had just been called".

This is true for the most part, but outbound RemoteRemoved HTLCs
were being serialized as normal, even though
`remote_uncommitted_htlcs_and_mark_paused` resets them to
`Committed`.

This led to a bug identified by the `chanmon_consistency_target`
fuzzer wherein, if we receive a update_*_htlc message bug not the
corresponding commitment_signed prior to a serialization roundtrip,
we'd force-close the channel due to the peer "attempting to
fail/claim an HTLC which was already failed/claimed".

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -4451,9 +4451,10 @@ impl<Signer: Sign> Writeable for Channel<Signer> {
 				&OutboundHTLCState::Committed => {
 					1u8.write(writer)?;
 				},
-				&OutboundHTLCState::RemoteRemoved(ref fail_reason) => {
-					2u8.write(writer)?;
-					fail_reason.write(writer)?;
+				&OutboundHTLCState::RemoteRemoved(_) => {
+					// Treat this as a Committed because we haven't received the CS - they'll
+					// resend the claim/fail on reconnect as we all (hopefully) the missing CS.
+					1u8.write(writer)?;
 				},
 				&OutboundHTLCState::AwaitingRemoteRevokeToRemove(ref fail_reason) => {
 					3u8.write(writer)?;