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.go

@@ -150,6 +150,14 @@ type WalletController interface {
 	// p2wsh, etc.
 	NewAddress(addrType AddressType, change bool) (btcutil.Address, error)
 
+	// LastUnusedAddress returns the last *unused* address known by the
+	// wallet. An address is unused if it hasn't received any payments.
+	// This can be useful in UIs in order to continually show the
+	// "freshest" address without having to worry about "address inflation"
+	// caused by continual refreshing. Similar to NewAddress it can derive
+	// a specified address type. By default, this is a non-change address.
+	LastUnusedAddress(addrType AddressType) (btcutil.Address, error)
+
 	// IsOurAddress checks if the passed address belongs to this wallet
 	IsOurAddress(a btcutil.Address) bool