Don’t trigger presence events on MUC presence

Specifically, previously, each MUC would be added as a roster item, and
then each join presence would be counted as a resource of that item,
triggering 1 to 5 events and more backend logic in slixmpp.

As a result, joining big rooms is tremendously slow, (JID() calls,
event() calls, __getitem__ calls for nothing), and takes RAM (a quick
tracemalloc tells me around 1 MiB for 3500 participants, i.e. 2 big IRC
rooms). Those resources may not necessarily be cleaned properly, leading
to memory leaks on long-term usage.

This is a micro-optimization that adds an attribute to roster items so
that MUC room events can be ignored safely while not affecting common
roster usage.

slixmpp/plugins/xep_0045.py

@@ -160,6 +160,7 @@ class XEP_0045(BasePlugin):
         got_online = False
         if pr['muc']['room'] not in self.rooms.keys():
             return
+        self.xmpp.roster[pr['from']].ignore_updates = True
         entry = pr['muc'].get_stanza_values()
         entry['show'] = pr['show']
         entry['status'] = pr['status']