Make the relay acknowledge all received commands

To that effect, bump liberty and the xC relay protocol version.
Relay events have been reordered to improve forward compatibility.

Also prevent use-after-free when serialization fails.

xP now slightly throttles activity notifications,
and indicates when there are unacknowledged commands.

xW/xW.cpp

@@ -221,6 +221,14 @@ relay_try_write(std::wstring &error)
 	return true;
 }
 
+static void
+on_relay_generic_response(
+	std::wstring error, const Relay::ResponseData *response)
+{
+	if (!response)
+		show_error_message(error.c_str());
+}
+
 static void
 relay_send(Relay::CommandData *data, Callback callback = {})
 {
@@ -232,6 +240,8 @@ relay_send(Relay::CommandData *data, Callback callback = {})
 
 	if (callback)
 		g.command_callbacks[m.command_seq] = std::move(callback);
+	else
+		g.command_callbacks[m.command_seq] = on_relay_generic_response;
 
 	uint32_t len = htonl(w.data.size());
 	uint8_t *prefix = reinterpret_cast<uint8_t *>(&len);