multi: address lingering TODO by no longer wiping out local HTLCs on remote close

In this commit, we fix a lingering TOOD statement in the channel arb.
Before this commitment, we would simply wipe our our local HTLC set of
the HTLC set that was on the remote commitment transaction on force
close. This was incorrect as if our commitment transaction had an HTLC
that the remote commitment didn't, then we would fail to cancel that
back, and cause both channels to time out on chain.

In order to remedy this, we introduce a new `HtlcSetKey` struct to track
all 3 possible in-flight set of HTLCs: ours, theirs, and their pending.

We also we start to tack on additional data to all the unilateral close
messages we send to subscribers. This new data is the CommitSet, or the
set of valid commitments at channel closure time. This new information
will be used by the channel arb in an upcoming commit to ensure it will
cancel back HTLCs in the case of split commitment state.

Finally, we start to thread through an optional *CommitSet to the
advanceState method. This additional information will give the channel
arb addition information it needs to ensure it properly cancels back
HTLCs that are about to time out or may time out depending on which
commitment is played.

Within the htlcswitch pakage, we modify the `SignNextCommitment` method
to return the new set of pending HTLCs for the remote party's commitment
transaction and `ReceiveRevocation` to return the latest set of
commitment transactions on the remote party's commitment as well. This
is a preparatory change which is part of a larger change to address a
lingering TODO in the cnct.

Additionally, rather than just send of the set of HTLCs after the we
revoke, we'll also send of the set of HTLCs after the remote party
revokes, and we create a pending commitment state for it.

contractcourt/chain_arbitrator.go

@@ -296,8 +296,25 @@ func newActiveChannelArbitrator(channel *channeldb.OpenChannel,
 		return c.resolveContract(chanPoint, chanLog)
 	}
 
+	// Finally, we'll need to construct a series of htlc Sets based on all
+	// currently known valid commitments.
+	htlcSets := make(map[HtlcSetKey]htlcSet)
+	htlcSets[LocalHtlcSet] = newHtlcSet(channel.LocalCommitment.Htlcs)
+	htlcSets[RemoteHtlcSet] = newHtlcSet(channel.RemoteCommitment.Htlcs)
+
+	pendingRemoteCommitment, err := channel.RemoteCommitChainTip()
+	if err != nil && err != channeldb.ErrNoPendingCommit {
+		blockEpoch.Cancel()
+		return nil, err
+	}
+	if pendingRemoteCommitment != nil {
+		htlcSets[RemotePendingHtlcSet] = newHtlcSet(
+			pendingRemoteCommitment.Commitment.Htlcs,
+		)
+	}
+
 	return NewChannelArbitrator(
-		arbCfg, channel.LocalCommitment.Htlcs, chanLog,
+		arbCfg, htlcSets, chanLog,
 	), nil
 }
 
@@ -557,15 +574,27 @@ func (c *ChainArbitrator) Stop() error {
 	return nil
 }
 
+// ContractUpdate is a message packages the latest set of active HTLCs on a
+// commitment, and also identifies which commitment received a new set of
+// HTLCs.
+type ContractUpdate struct {
+	// HtlcKey identifies which commitment the HTLCs below are present on.
+	HtlcKey HtlcSetKey
+
+	// Htlcs are the of active HTLCs on the commitment identified by the
+	// above HtlcKey.
+	Htlcs []channeldb.HTLC
+}
+
 // ContractSignals wraps the two signals that affect the state of a channel
 // being watched by an arbitrator. The two signals we care about are: the
 // channel has a new set of HTLC's, and the remote party has just broadcast
 // their version of the commitment transaction.
 type ContractSignals struct {
-	// HtlcUpdates is a channel that once we new commitment updates takes
-	// place, the later set of HTLC's on the commitment transaction should
-	// be sent over.
-	HtlcUpdates chan []channeldb.HTLC
+	// HtlcUpdates is a channel that the link will use to update the
+	// designated channel arbitrator when the set of HTLCs on any valid
+	// commitment changes.
+	HtlcUpdates chan *ContractUpdate
 
 	// ShortChanID is the up to date short channel ID for a contract. This
 	// can change either if when the contract was added it didn't yet have