funding: remove dead code and sanity check pending chan ID (#7887)

* funding: remove unused field `newChanBarriers`

This commit removes the occurance of `newChanBarriers` as it's not used
anywhere.

* funding: rename method names to clear the funding flow

Slightly refactored the names so it's easier to see which side is
processing what messages.

* lnwallet: sanity check empty pending channel ID

This commit adds a sanity check to make sure an empty pending channel ID
will not be accepted.

lnwallet/wallet_test.go

@@ -0,0 +1,34 @@
+package lnwallet
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestRegisterFundingIntent checks RegisterFundingIntent behaves as expected.
+func TestRegisterFundingIntent(t *testing.T) {
+	t.Parallel()
+
+	require := require.New(t)
+
+	// Create a testing wallet.
+	lw, err := NewLightningWallet(Config{})
+	require.NoError(err)
+
+	// Init an empty testing channel ID.
+	var testID [32]byte
+
+	// Call the method with empty ID should give us an error.
+	err = lw.RegisterFundingIntent(testID, nil)
+	require.ErrorIs(err, ErrEmptyPendingChanID)
+
+	// Modify the ID and call the method again should result in no error.
+	testID[0] = 1
+	err = lw.RegisterFundingIntent(testID, nil)
+	require.NoError(err)
+
+	// Call the method using the same ID should give us an error.
+	err = lw.RegisterFundingIntent(testID, nil)
+	require.ErrorIs(err, ErrDuplicatePendingChanID)
+}