Do not process MAM messages in main message handle

Since all handlers get called for every stanza, even though the
MessageSync object has its own handler, this event still fires, but we
want to do as little work as possible and bail out since we know it's
being handled elsewhere.

snikket/Client.hx

@@ -135,6 +135,12 @@ class Client extends EventEmitter {
 
 		stream.on("message", function(event) {
 			final stanza:Stanza = event.stanza;
+
+			if (stanza.getChild("result", "urn:xmpp:mam:2") != null) {
+				// We don't want to process MAM messages here
+				return EventUnhandled;
+			}
+
 			final from = stanza.attr.get("from") == null ? null : JID.parse(stanza.attr.get("from"));
 
 			if (stanza.attr.get("type") == "error" && from != null) {