lnd+server+watchtower: allow Standalone access to the tor controller

This commit lets the watchtower automatically create hidden services
by giving it a pointer to a TorController. The server was also slightly
refactored so that it was not the sole owner of the TorController.

watchtower/config.go

@@ -34,8 +34,8 @@ var (
 )
 
 // Config defines the resources and parameters used to configure a Watchtower.
-// All nil-able elements with the Config must be set in order for the Watchtower
-// to function properly.
+// All nil-able elements besides tor-related ones must be set in order for the
+// Watchtower to function properly.
 type Config struct {
 	// ChainHash identifies the chain that the watchtower will be monitoring
 	// for breaches and that will be advertised in the server's Init message
@@ -89,4 +89,16 @@ type Config struct {
 	// message from the other end, if the connection has stopped buffering
 	// the server's replies.
 	WriteTimeout time.Duration
+
+	// TorController allows the watchtower to optionally setup an onion hidden
+	// service.
+	TorController *tor.Controller
+
+	// WatchtowerKeyPath allows the watchtower to specify where the private key
+	// for a watchtower hidden service should be stored.
+	WatchtowerKeyPath string
+
+	// Type specifies the hidden service type (V2 or V3) that the watchtower
+	// will create.
+	Type tor.OnionType
 }