lnwallet: add LastUnusedAddress to WalletController interface

In this commit, we add a new `LastUnusedAddress` method to the
`WalletController` interface. Callers can use this new method to graph
the last unused address, which can be useful for UIs that want to
refresh the address, but not cause nearly unbounded address generation.

The implementation for `btcwallet` uses the existing `CurrentAddress`
method. We've also added a new integration tests to exercise the new
functionality.

lnwallet/interface_test.go

@@ -2165,6 +2165,62 @@ func testChangeOutputSpendConfirmation(r *rpctest.Harness,
 	}
 }
 
+// testLastUnusedAddr tests that the LastUnusedAddress returns the address if
+// it isn't used, and also that once the address becomes used, then it's
+// properly rotated.
+func testLastUnusedAddr(miner *rpctest.Harness,
+	alice, bob *lnwallet.LightningWallet, t *testing.T) {
+
+	if _, err := miner.Node.Generate(1); err != nil {
+		t.Fatalf("unable to generate block: %v", err)
+	}
+
+	// We'll repeat this test for each address type to ensure they're all
+	// rotated properly.
+	addrTypes := []lnwallet.AddressType{
+		lnwallet.WitnessPubKey, lnwallet.NestedWitnessPubKey,
+	}
+	for _, addrType := range addrTypes {
+		addr1, err := alice.LastUnusedAddress(addrType)
+		if err != nil {
+			t.Fatalf("unable to get addr: %v", err)
+		}
+		addr2, err := alice.LastUnusedAddress(addrType)
+		if err != nil {
+			t.Fatalf("unable to get addr: %v", err)
+		}
+
+		// If we generate two addresses back to back, then we should
+		// get the same addr, as none of them have been used yet.
+		if addr1.String() != addr2.String() {
+			t.Fatalf("addresses changed w/o use: %v vs %v", addr1, addr2)
+		}
+
+		// Next, we'll have Bob pay to Alice's new address. This should
+		// trigger address rotation at the backend wallet.
+		addrScript, err := txscript.PayToAddrScript(addr1)
+		if err != nil {
+			t.Fatalf("unable to convert addr to script: %v", err)
+		}
+		feeRate := lnwallet.SatPerKWeight(2500)
+		output := &wire.TxOut{
+			Value:    1000000,
+			PkScript: addrScript,
+		}
+		sendCoins(t, miner, bob, alice, output, feeRate)
+
+		// If we make a new address, then it should be brand new, as
+		// the prior address has been used.
+		addr3, err := alice.LastUnusedAddress(addrType)
+		if err != nil {
+			t.Fatalf("unable to get addr: %v", err)
+		}
+		if addr1.String() == addr3.String() {
+			t.Fatalf("address should have changed but didn't")
+		}
+	}
+}
+
 type walletTestCase struct {
 	name string
 	test func(miner *rpctest.Harness, alice, bob *lnwallet.LightningWallet,
@@ -2219,6 +2275,10 @@ var walletTests = []walletTestCase{
 		name: "test cancel non-existent reservation",
 		test: testCancelNonExistentReservation,
 	},
+	{
+		name: "last unused addr",
+		test: testLastUnusedAddr,
+	},
 	{
 		name: "reorg wallet balance",
 		test: testReorgWalletBalance,