lnwallet: fix race condition when forwarding by creating new pubkey

This commit fixes a race condition that would at times occur in the
htlcswitch.TestChannelLinkBidirectionalOneHopPayments test case. A race
condition would occur in the goroutine running ReceiveNewCommitment
compared with the grouting that would obtain the snapshot in order to
make a forwarding decision.

We fix this by creating a new public key for each new commitment
transaction such that we complete avoid the read/write race condition.

lnwallet/channel.go

@@ -1838,11 +1838,15 @@ func (lc *LightningChannel) ReceiveNewCommitment(rawSig []byte) error {
 
 	// Ensure that the newly constructed commitment state has a valid
 	// signature.
-	theirMultiSigKey.Curve = btcec.S256()
+	verifyKey := btcec.PublicKey{
+		X:     theirMultiSigKey.X,
+		Y:     theirMultiSigKey.Y,
+		Curve: btcec.S256(),
+	}
 	sig, err := btcec.ParseSignature(rawSig, btcec.S256())
 	if err != nil {
 		return err
-	} else if !sig.Verify(sigHash, theirMultiSigKey) {
+	} else if !sig.Verify(sigHash, &verifyKey) {
 		return fmt.Errorf("invalid commitment signature")
 	}