xC/xP: only send buffer stats in the initial sync

The client and frontends track these separately,
there is no need for hard synchronization.

xC-proto

@@ -19,7 +19,7 @@ struct CommandMessage {
 	case HELLO:
 		u32 version;
 		// If the version check succeeds, the client will receive
-		// an initial stream of BUFFER_UPDATE, BUFFER_LINE,
+		// an initial stream of BUFFER_UPDATE, BUFFER_STATS, BUFFER_LINE,
 		// and finally a BUFFER_ACTIVATE message.
 	case ACTIVE:
 		void;
@@ -50,6 +50,7 @@ struct EventMessage {
 	union EventData switch (enum Event {
 		PING,
 		BUFFER_UPDATE,
+		BUFFER_STATS,
 		BUFFER_RENAME,
 		BUFFER_REMOVE,
 		BUFFER_ACTIVATE,
@@ -62,12 +63,13 @@ struct EventMessage {
 		void;
 	case BUFFER_UPDATE:
 		string buffer_name;
+	case BUFFER_STATS:
+		string buffer_name;
 		// These are cumulative, even for lines flushed out from buffers.
 		// Updates to these values aren't broadcasted, thus handle:
 		//  - BUFFER_LINE by bumping/setting them as appropriate,
 		//  - BUFFER_ACTIVATE by clearing them for the previous buffer
 		//    (this way, they can be used to mark unread messages).
-		// Any updates received after the initial sync should be ignored.
 		u32 new_messages;
 		u32 new_unimportant_messages;
 		bool highlighted;