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.

xA/xA.go

@@ -337,9 +337,14 @@ func relaySend(data RelayCommandData, callback callback) bool {
 		CommandSeq: commandSeq,
 		Data:       data,
 	}
-	if callback != nil {
-		commandCallbacks[m.CommandSeq] = callback
+	if callback == nil {
+		callback = func(err string, response *RelayResponseData) {
+			if response == nil {
+				showErrorMessage(err)
+			}
+		}
 	}
+	commandCallbacks[m.CommandSeq] = callback
 	commandSeq++
 
 	// TODO(p): Handle errors better.