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.

xM/main.swift

@@ -173,8 +173,11 @@ class RelayRPC {
 	func send(data: RelayCommandData, callback: Callback? = nil) {
 		self.commandSeq += 1
 		let m = RelayCommandMessage(commandSeq: self.commandSeq, data: data)
-		if let callback = callback {
-			self.commandCallbacks[m.commandSeq] = callback
+		self.commandCallbacks[m.commandSeq] = callback ?? { error, data in
+			if data == nil {
+				NSSound.beep()
+				Logger().warning("\(error)")
+			}
 		}
 
 		var w = RelayWriter()