mod_mam: Suppress offline message broadcast for MAM clients

MattJ on 09:34:24
> Zash: I think as a first step, offline messages should not be sent to
> clients that request MAM

https://chat.modernxmpp.org/log/modernxmpp/2021-08-31#2021-08-31-8518a542bd283686

CHANGES

@@ -27,6 +27,7 @@ TRUNK
 -   mod_server_contact_info now loaded on components if enabled
 -   Statistics now based on OpenMetrics
 -   Direct TLS support
+-   Offline messages aren't sent to MAM clients
 
 0.11.0
 ======

plugins/mod_mam/mod_mam.lua

@@ -488,6 +488,13 @@ module:hook("message/offline/handle", function(event)
 	end
 end, -2);
 
+-- Don't broadcast offline messages to clients that have queried the archive.
+module:hook("message/offline/broadcast", function (event)
+	if event.origin.mam_requested then
+		return true;
+	end
+end);
+
 if cleanup_after ~= "never" then
 	local cleanup_storage = module:open_store("archive_cleanup");
 	local cleanup_map = module:open_store("archive_cleanup", "map");