chancloser: new package for cooperative channel closure

Introduces a new chancloser package which exposes a ChanCloser
struct that handles the cooperative channel closure negotiation
and is meant to replace chancloser.go in the lnd package. Updates
all references to chancloser.go to instead use chancloser package.

lnwallet/chancloser/chancloser_test.go

@@ -0,0 +1,76 @@
+package chancloser
+
+import (
+	"crypto/rand"
+	"testing"
+
+	"github.com/lightningnetwork/lnd/lnwire"
+)
+
+// randDeliveryAddress generates a random delivery address for testing.
+func randDeliveryAddress(t *testing.T) lnwire.DeliveryAddress {
+	// Generate an address of maximum length.
+	da := lnwire.DeliveryAddress(make([]byte, 34))
+
+	_, err := rand.Read(da)
+	if err != nil {
+		t.Fatalf("cannot generate random address: %v", err)
+	}
+
+	return da
+}
+
+// TestMaybeMatchScript tests that the maybeMatchScript errors appropriately
+// when an upfront shutdown script is set and the script provided does not
+// match, and does not error in any other case.
+func TestMaybeMatchScript(t *testing.T) {
+	addr1 := randDeliveryAddress(t)
+	addr2 := randDeliveryAddress(t)
+
+	tests := []struct {
+		name           string
+		shutdownScript lnwire.DeliveryAddress
+		upfrontScript  lnwire.DeliveryAddress
+		expectedErr    error
+	}{
+		{
+			name:           "no upfront shutdown set, script ok",
+			shutdownScript: addr1,
+			upfrontScript:  []byte{},
+			expectedErr:    nil,
+		},
+		{
+			name:           "upfront shutdown set, script ok",
+			shutdownScript: addr1,
+			upfrontScript:  addr1,
+			expectedErr:    nil,
+		},
+		{
+			name:           "upfront shutdown set, script not ok",
+			shutdownScript: addr1,
+			upfrontScript:  addr2,
+			expectedErr:    ErrUpfrontShutdownScriptMismatch,
+		},
+		{
+			name:           "nil shutdown and empty upfront",
+			shutdownScript: nil,
+			upfrontScript:  []byte{},
+			expectedErr:    nil,
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+
+		t.Run(test.name, func(t *testing.T) {
+			err := maybeMatchScript(
+				func() error { return nil }, test.upfrontScript,
+				test.shutdownScript,
+			)
+
+			if err != test.expectedErr {
+				t.Fatalf("Error: %v, expected error: %v", err, test.expectedErr)
+			}
+		})
+	}
+}