Change session_start callback to async in most examples

If we fetch the roster, we should probably wait until we get it back

examples/pubsub_client.py

@@ -32,7 +32,7 @@ class PubsubClient(slixmpp.ClientXMPP):
         self.add_event_handler('session_start', self.start)
 
     async def start(self, event):
-        self.get_roster()
+        await self.get_roster()
         self.send_presence()
 
         try: