lnwallet+contractcourt: publicly export DeriveCommitmentKeys use in chain watcher

In this commit, we consolidate the number of areas where we derive our
commitment keys. Before this commit, the `isOurCommitment` function in
the chain watcher used a custom routine to derive the expected
scripts/keys for our commitment at that height. With the recent changes,
we now have additional logic in `DeriveCommitmentKeys` that wasn't
copied over to this area. As a result, the prior logic would erroneously
detect if it was our commitment that had hit the chain or not.

In this commit, we remove the old custom code, and use
`DeriveCommitmentKeys` wihtin the chain watcher as well. This ensures
that we only need to maintain the key derivation code in a single place,
preventing future bugs of this nature.

contractcourt/chain_watcher.go

@@ -331,7 +331,7 @@ func (c *chainWatcher) SubscribeChannelEvents() *ChainEventSubscription {
 // based off of only the set of outputs included.
 func isOurCommitment(localChanCfg, remoteChanCfg channeldb.ChannelConfig,
 	commitSpend *chainntnfs.SpendDetail, broadcastStateNum uint64,
-	revocationProducer shachain.Producer) (bool, error) {
+	revocationProducer shachain.Producer, tweakless bool) (bool, error) {
 
 	// First, we'll re-derive our commitment point for this state since
 	// this is what we use to randomize each of the keys for this state.
@@ -344,14 +344,15 @@ func isOurCommitment(localChanCfg, remoteChanCfg channeldb.ChannelConfig,
 	// Now that we have the commit point, we'll derive the tweaked local
 	// and remote keys for this state. We use our point as only we can
 	// revoke our own commitment.
-	localDelayBasePoint := localChanCfg.DelayBasePoint.PubKey
-	localDelayKey := input.TweakPubKey(localDelayBasePoint, commitPoint)
-	remoteNonDelayPoint := remoteChanCfg.PaymentBasePoint.PubKey
-	remotePayKey := input.TweakPubKey(remoteNonDelayPoint, commitPoint)
+	commitKeyRing := lnwallet.DeriveCommitmentKeys(
+		commitPoint, true, tweakless, &localChanCfg, &remoteChanCfg,
+	)
 
 	// With the keys derived, we'll construct the remote script that'll be
 	// present if they have a non-dust balance on the commitment.
-	remotePkScript, err := input.CommitScriptUnencumbered(remotePayKey)
+	remotePkScript, err := input.CommitScriptUnencumbered(
+		commitKeyRing.NoDelayKey,
+	)
 	if err != nil {
 		return false, err
 	}
@@ -359,11 +360,9 @@ func isOurCommitment(localChanCfg, remoteChanCfg channeldb.ChannelConfig,
 	// Next, we'll derive our script that includes the revocation base for
 	// the remote party allowing them to claim this output before the CSV
 	// delay if we breach.
-	revocationKey := input.DeriveRevocationPubkey(
-		remoteChanCfg.RevocationBasePoint.PubKey, commitPoint,
-	)
 	localScript, err := input.CommitScriptToSelf(
-		uint32(localChanCfg.CsvDelay), localDelayKey, revocationKey,
+		uint32(localChanCfg.CsvDelay), commitKeyRing.DelayKey,
+		commitKeyRing.RevocationKey,
 	)
 	if err != nil {
 		return false, err
@@ -423,6 +422,11 @@ func (c *chainWatcher) closeObserver(spendNtfn *chainntnfs.SpendEvent) {
 		// revoked state...!!!
 		commitTxBroadcast := commitSpend.SpendingTx
 
+		// An additional piece of information we need to properly
+		// dispatch a close event if is this channel was using the
+		// tweakless remove key format or not.
+		tweaklessCommit := c.cfg.chanState.ChanType.IsTweakless()
+
 		localCommit, remoteCommit, err := c.cfg.chanState.LatestCommitments()
 		if err != nil {
 			log.Errorf("Unable to fetch channel state for "+
@@ -480,6 +484,7 @@ func (c *chainWatcher) closeObserver(spendNtfn *chainntnfs.SpendEvent) {
 			c.cfg.chanState.LocalChanCfg,
 			c.cfg.chanState.RemoteChanCfg, commitSpend,
 			broadcastStateNum, c.cfg.chanState.RevocationProducer,
+			tweaklessCommit,
 		)
 		if err != nil {
 			log.Errorf("unable to determine self commit for "+
@@ -584,9 +589,6 @@ func (c *chainWatcher) closeObserver(spendNtfn *chainntnfs.SpendEvent) {
 				"state #%v!!! Attempting recovery...",
 				broadcastStateNum, remoteStateNum)
 
-			tweaklessCommit := (c.cfg.chanState.ChanType ==
-				channeldb.SingleFunderTweakless)
-
 			// If this isn't a tweakless commitment, then we'll
 			// need to wait for the remote party's latest unrevoked
 			// commitment point to be presented to us as we need