patch 7.4.1623
Problem:    All Channels share the message ID, it keeps getting bigger.
Solution:   Use a message ID per channel.
diff --git a/src/structs.h b/src/structs.h
index 470beff..85223a0 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1380,6 +1380,7 @@
     channel_T	*ch_prev;
 
     int		ch_id;		/* ID of the channel */
+    int		ch_last_msg_id;	/* ID of the last message */
 
     chanpart_T	ch_part[4];	/* info for socket, out, err and in */