lnd: fix Onion v2 support for Neutrino backends

With this commit we make it possible to use an Onion v2 hidden service
address as the Neutrino backend.
This failed before because an .onion address cannot be looked up and
converted into an IP address through the normal DNS resolving process,
even when using a Tor socks proxy.
Instead, we turn any v2 .onion address into a fake IPv6 representation
before giving it to Neutrino's address manager and turn it back into an
Onion host address when actually dialing.

lnd.go

@@ -1508,12 +1508,43 @@ func initNeutrinoBackend(cfg *Config, chainDir string) (*neutrino.ChainService,
 		AddPeers:     cfg.NeutrinoMode.AddPeers,
 		ConnectPeers: cfg.NeutrinoMode.ConnectPeers,
 		Dialer: func(addr net.Addr) (net.Conn, error) {
+			dialAddr := addr
+			if tor.IsOnionFakeIP(addr) {
+				// Because the Neutrino address manager only
+				// knows IP addresses, we need to turn any fake
+				// tcp6 address that actually encodes an Onion
+				// v2 address back into the hostname
+				// representation before we can pass it to the
+				// dialer.
+				var err error
+				dialAddr, err = tor.FakeIPToOnionHost(addr)
+				if err != nil {
+					return nil, err
+				}
+			}
+
 			return cfg.net.Dial(
-				addr.Network(), addr.String(),
+				dialAddr.Network(), dialAddr.String(),
 				cfg.ConnectionTimeout,
 			)
 		},
 		NameResolver: func(host string) ([]net.IP, error) {
+			if tor.IsOnionHost(host) {
+				// Neutrino internally uses btcd's address
+				// manager which only operates on an IP level
+				// and does not understand onion hosts. We need
+				// to turn an onion host into a fake
+				// representation of an IP address to make it
+				// possible to connect to a block filter backend
+				// that serves on an Onion v2 hidden service.
+				fakeIP, err := tor.OnionHostToFakeIP(host)
+				if err != nil {
+					return nil, err
+				}
+
+				return []net.IP{fakeIP}, nil
+			}
+
 			addrs, err := cfg.net.LookupHost(host)
 			if err != nil {
 				return nil, err