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.

xC.lxdr

@@ -1,7 +1,8 @@
 // Backwards-compatible protocol version.
-const VERSION = 1;
+const VERSION = 2;
 
 // From the frontend to the relay.
+// All commands receive either an Event.RESPONSE, or an Event.ERROR.
 struct CommandMessage {
 	// The command sequence number will be repeated in responses
 	// in the respective fields.
@@ -32,13 +33,10 @@ struct CommandMessage {
 	// XXX: Perhaps this should rather be handled through a /buffer command.
 	case BUFFER_TOGGLE_UNIMPORTANT:
 		string buffer_name;
-	case PING_RESPONSE:
-		u32 event_seq;
-
-	// Only these commands may produce Event.RESPONSE, as below,
-	// but any command may produce an error.
 	case PING:
 		void;
+	case PING_RESPONSE:
+		u32 event_seq;
 	case BUFFER_COMPLETE:
 		string buffer_name;
 		string text;
@@ -52,6 +50,9 @@ struct CommandMessage {
 struct EventMessage {
 	u32 event_seq;
 	union EventData switch (enum Event {
+		ERROR,
+		RESPONSE,
+
 		PING,
 		BUFFER_LINE,
 		BUFFER_UPDATE,
@@ -64,12 +65,28 @@ struct EventMessage {
 		SERVER_UPDATE,
 		SERVER_RENAME,
 		SERVER_REMOVE,
-		ERROR,
-		RESPONSE,
 	} event) {
+	// Restriction: command_seq strictly follows the sequence received
+	// by the relay, across both of these replies.
+	case ERROR:
+		u32 command_seq;
+		string error;
+	case RESPONSE:
+		u32 command_seq;
+		union ResponseData switch (Command command) {
+		case BUFFER_COMPLETE:
+			u32 start;
+			string completions<>;
+		case BUFFER_LOG:
+			// UTF-8, but not guaranteed.
+			u8 log<>;
+		default:
+			// Reception acknowledged.
+			void;
+		} data;
+
 	case PING:
 		void;
-
 	case BUFFER_LINE:
 		string buffer_name;
 		// Whether the line should also be displayed in the active buffer.
@@ -188,23 +205,5 @@ struct EventMessage {
 		string new;
 	case SERVER_REMOVE:
 		string server_name;
-
-	// Restriction: command_seq strictly follows the sequence received
-	// by the relay, across both of these replies.
-	case ERROR:
-		u32 command_seq;
-		string error;
-	case RESPONSE:
-		u32 command_seq;
-		union ResponseData switch (Command command) {
-		case PING:
-			void;
-		case BUFFER_COMPLETE:
-			u32 start;
-			string completions<>;
-		case BUFFER_LOG:
-			// UTF-8, but not guaranteed.
-			u8 log<>;
-		} data;
 	} data;
 };