plugin pubsub cache: Pubsub Caching implementation

sat/plugins/plugin_xep_0060.py

@@ -91,6 +91,8 @@ class XEP_0060(object):
     ID_SINGLETON = "current"
     EXTRA_PUBLISH_OPTIONS = "publish_options"
     EXTRA_ON_PRECOND_NOT_MET = "on_precondition_not_met"
+    # extra disco needed for RSM, cf. XEP-0060 § 6.5.4
+    DISCO_RSM = "http://jabber.org/protocol/pubsub#rsm"
 
     def __init__(self, host):
         log.info(_("PubSub plugin initialization"))
@@ -392,6 +394,7 @@ class XEP_0060(object):
             the method must be named after PubSub constants in lower case
             and suffixed with "_cb"
             e.g.: "items_cb" for C.PS_ITEMS, "delete_cb" for C.PS_DELETE
+            note: only C.PS_ITEMS and C.PS_DELETE are implemented so far
         """
         assert node is not None
         assert kwargs