lnwallet+htlcswitch: add NewCommitState struct, modify send/recv sig to accept

In this commit, we add a new NewCommitState struct. This preps us for
the future change wherein a partial signature is also added to the mix.
All related tests and type signatures have also been updated
accordingly.

htlcswitch/link_isolated_test.go

@@ -93,14 +93,14 @@ func (l *linkTestContext) receiveHtlcAliceToBob() {
 func (l *linkTestContext) sendCommitSigBobToAlice(expHtlcs int) {
 	l.t.Helper()
 
-	sig, htlcSigs, _, err := l.bobChannel.SignNextCommitment()
+	sigs, err := l.bobChannel.SignNextCommitment()
 	if err != nil {
 		l.t.Fatalf("error signing commitment: %v", err)
 	}
 
 	commitSig := &lnwire.CommitSig{
-		CommitSig: sig,
-		HtlcSigs:  htlcSigs,
+		CommitSig: sigs.CommitSig,
+		HtlcSigs:  sigs.HtlcSigs,
 	}
 
 	if len(commitSig.HtlcSigs) != expHtlcs {
@@ -141,9 +141,10 @@ func (l *linkTestContext) receiveCommitSigAliceToBob(expHtlcs int) {
 
 	comSig := l.receiveCommitSigAlice(expHtlcs)
 
-	err := l.bobChannel.ReceiveNewCommitment(
-		comSig.CommitSig, comSig.HtlcSigs,
-	)
+	err := l.bobChannel.ReceiveNewCommitment(&lnwallet.CommitSigs{
+		CommitSig: comSig.CommitSig,
+		HtlcSigs:  comSig.HtlcSigs,
+	})
 	if err != nil {
 		l.t.Fatalf("bob failed receiving commitment: %v", err)
 	}