server: add watchtower client

watchtower/wtclient/client.go

@@ -29,6 +29,11 @@ const (
 	// DefaultStatInterval specifies the default interval between logging
 	// metrics about the client's operation.
 	DefaultStatInterval = 30 * time.Second
+
+	// DefaultForceQuitDelay specifies the default duration after which the
+	// client should abandon any pending updates or session negotiations
+	// before terminating.
+	DefaultForceQuitDelay = 10 * time.Second
 )
 
 // Client is the primary interface used by the daemon to control a client's